diff --git a/lib/jeff.ex b/lib/jeff.ex
index a6e409d..90264b1 100644
--- a/lib/jeff.ex
+++ b/lib/jeff.ex
@@ -1,47 +1,80 @@
 defmodule Jeff do
-  alias Jeff.ACU
+  @moduledoc """
+  Control an Access Control Unit (ACU) and send commands to a Peripheral Device (PD)
+  """
 
+  alias Jeff.{ACU, Command, Device, Reply}
+
+  @type acu() :: GenServer.server()
+  @type device_opt() :: ACU.device_opt()
+  @type osdp_address() :: 0x00..0x7F
+
+  @doc """
+  Start an ACU process.
+  """
+  @spec start_acu([ACU.start_opt()]) :: GenServer.on_start()
   def start_acu(opts \\ []) do
     ACU.start_link(opts)
   end
 
+  @doc """
+  Register a peripheral device on the ACU communication bus.
+  """
+  @spec add_pd(acu(), osdp_address(), [device_opt()]) :: Device.t()
   def add_pd(acu, address, opts \\ []) do
     ACU.add_device(acu, address, opts)
   end
 
+  @doc """
+  Requests the return of the PD ID Report.
+  """
+  @spec id_report(acu(), osdp_address()) :: Reply.IdReport.t() | Reply.ErrorCode.t()
   def id_report(acu, address) do
-    ACU.send_command(acu, address, ID)
+    ACU.send_command(acu, address, ID).data
   end
 
+  @doc """
+  Requests the PD to return a list of its functional capabilities, such as the
+  type and number of input points, outputs points, reader ports, etc.
+  """
+  @spec capabilities(acu(), osdp_address()) :: [Reply.Capability.t()] | Reply.ErrorCode.t()
   def capabilities(acu, address) do
-    ACU.send_command(acu, address, CAP)
+    ACU.send_command(acu, address, CAP).data
   end
 
+  @doc """
+  Instructs the PD to reply with a local status report.
+  """
+  @spec local_status(acu(), osdp_address()) :: Reply.LocalStatus.t() | Reply.ErrorCode.t()
   def local_status(acu, address) do
-    ACU.send_command(acu, address, LSTAT)
-  end
-
-  def input_status(acu, address) do
-    ACU.send_command(acu, address, ISTAT)
+    ACU.send_command(acu, address, LSTAT).data
   end
 
+  @doc """
+  Controls the LEDs associated with one or more readers.
+  """
+  @spec set_led(acu(), osdp_address(), Command.LedSettings.params()) ::
+          Reply.ACK | Reply.ErrorCode.t()
   def set_led(acu, address, params) do
-    ACU.send_command(acu, address, LED, params)
+    ACU.send_command(acu, address, LED, params).data
   end
 
+  @doc """
+  Defines commands to a single, monotone audible annunciator (beeper or buzzer)
+  that may be associated with a reader.
+  """
+  @spec set_buzzer(acu(), osdp_address(), Command.BuzzerSettings.params()) ::
+          Reply.ACK | Reply.ErrorCode.t()
   def set_buzzer(acu, address, params) do
-    ACU.send_command(acu, address, BUZ, params)
+    ACU.send_command(acu, address, BUZ, params).data
   end
 
+  @doc """
+  Sets the PD's communication parameters.
+  """
+  @spec set_com(acu(), osdp_address(), Command.ComSettings.params()) ::
+          Reply.ComData.t() | Reply.ErrorCode.t()
   def set_com(acu, address, params) do
-    ACU.send_command(acu, address, COMSET, params)
-  end
-
-  def set_key(acu, address, params) do
-    ACU.send_command(acu, address, KEYSET, params)
-  end
-
-  def abort(acu, address) do
-    ACU.send_command(acu, address, ABORT)
+    ACU.send_command(acu, address, COMSET, params).data
   end
 end
diff --git a/lib/jeff/acu.ex b/lib/jeff/acu.ex
index 8d9a26d..2c3b8e6 100644
--- a/lib/jeff/acu.ex
+++ b/lib/jeff/acu.ex
@@ -1,4 +1,8 @@
 defmodule Jeff.ACU do
+  @moduledoc """
+  GenServer process for an ACU
+  """
+
   require Logger
 
   use GenServer
@@ -6,37 +10,56 @@ defmodule Jeff.ACU do
 
   @max_reply_delay 200
 
-  @type osdp_address :: 0x0..0x7F
+  @type acu() :: Jeff.acu()
   @type address_availability :: :available | :registered | :timeout | :error
+  @type osdp_address() :: Jeff.osdp_address()
 
   @type start_opt() ::
-          {:name, :atom}
+          {:name, atom()}
           | {:serial_port, String.t()}
 
+  @type device_opt() :: {:check_scheme, atom()}
+
+  @doc """
+  Start the ACU process.
+  """
   @spec start_link([start_opt()]) :: GenServer.on_start()
   def start_link(opts \\ []) do
     {name, opts} = Keyword.pop(opts, :name)
     GenServer.start_link(__MODULE__, opts, name: name)
   end
 
-  def add_device(pid, address, opts \\ []) do
-    GenServer.call(pid, {:add_device, address, opts})
+  @doc """
+  Register a peripheral device on the ACU communication bus.
+  """
+  @spec add_device(acu(), osdp_address(), [device_opt()]) :: Device.t()
+  def add_device(acu, address, opts \\ []) do
+    GenServer.call(acu, {:add_device, address, opts})
   end
 
-  def send_command(pid, address, name, params \\ []) do
-    GenServer.call(pid, {:send_command, address, name, params})
+  @doc """
+  Send a command to a peripheral device.
+  """
+  @spec send_command(acu(), osdp_address(), atom(), keyword()) :: Reply.t()
+  def send_command(acu, address, name, params \\ []) do
+    GenServer.call(acu, {:send_command, address, name, params})
   end
 
-  def send_command_oob(pid, address, name, params \\ []) do
-    GenServer.call(pid, {:send_command_oob, address, name, params})
+  @doc """
+  Send a command to a peripheral device that is not yet registered on the ACU.
+  Intended to be used for maintenance/diagnostic purposes.
+  """
+  @spec send_command_oob(acu(), osdp_address(), atom(), keyword()) :: Reply.t()
+  def send_command_oob(acu, address, name, params \\ []) do
+    GenServer.call(acu, {:send_command_oob, address, name, params})
   end
 
   @doc """
   Determine if a device is available to be registered on the bus.
   """
-  @spec check_address(GenServer.server(), osdp_address()) :: address_availability()
-  def check_address(pid, address) do
-    GenServer.call(pid, {:check_address, address})
+  @spec check_address(acu(), osdp_address()) :: address_availability()
+  def check_address(acu, address) do
+    GenServer.call(acu, {:check_address, address})
   end
 
   @impl GenServer
diff --git a/lib/jeff/bus.ex b/lib/jeff/bus.ex
index acecffe..4a609bb 100644
--- a/lib/jeff/bus.ex
+++ b/lib/jeff/bus.ex
@@ -1,4 +1,6 @@
 defmodule Jeff.Bus do
+  @moduledoc false
+
   alias Jeff.Device
 
   defstruct registry: %{},
diff --git a/lib/jeff/command.ex b/lib/jeff/command.ex
index 1225de6..2c3e8f4 100644
--- a/lib/jeff/command.ex
+++ b/lib/jeff/command.ex
@@ -1,32 +1,35 @@
 defmodule Jeff.Command do
   @moduledoc """
-  Code | Name         | Description                           | Data Type
-  0x60 | POLL         | Poll                                  | -
-  0x61 | ID           | ID Report Request                     | -
-  0x62 | CAP          | PD Capabilities Request               | [Capability]
-  0x64 | LSTAT        | Local Status Report Request           | -
-  0x65 | ISTAT        | Input Status Report Request           | -
-  0x66 | OSTAT        | Output Status Report Request          | -
-  0x67 | RSTAT        | Reader Status Report Request          | -
-  0x68 | OUT          | Output Control Command                | OutputSettings
-  0x69 | LED          | Reader Led Control Command            | LedSettings
-  0x6A | BUZ          | Reader Buzzer Control Command         | BuzzerSettings
-  0x6B | TEXT         | Text Output Command                   | TextSettings
-  0x6E | COMSET       | PD Communication Config Command       | ComSettings
-  0x73 | BIOREAD      | Scan and Send Biometric Data          | Requested Return Format
-  0x74 | BIOMATCH     | Scan and Match Biometric Template     | Biometric Template
-  0x75 | KEYSET       | Encryption Key Set Command            | EncryptionKey
-  0x76 | CHLNG        | Challenge/Secure Session Init Request | ChallengeData
-  0x77 | SCRYPT       | Server Cryptogram                     | EncryptionData
-  0x7B | ACURXSIZE    | Max ACU receive size                  | Buffer size
-  0x7C | FILETRANSFER | Send data file to PD                  | File contents
-  0x80 | MFG          | Manufacturer Specific Command         | Any
-  0xA1 | XWR          | Extended write data                   | APDU and details
-  0xA2 | ABORT        | Abort PD operation                    | -
-  0xA3 | PIVDATA      | Get PIV Data                          | Object details
-  0xA4 | GENAUTH      | Request Authenticate                  | Request details
-  0xA5 | CRAUTH       | Request Crypto Response               | Challenge details
-  0xA7 | KEEPACTIVE   | PD read activation                    | Time duration
+  Commands are sent from an ACU to a PD
+
+  | Code | Name         | Description                           | Data Type               |
+  |------|--------------|---------------------------------------|-------------------------|
+  | 0x60 | POLL         | Poll                                  | -                       |
+  | 0x61 | ID           | ID Report Request                     | -                       |
+  | 0x62 | CAP          | PD Capabilities Request               | [Capability]            |
+  | 0x64 | LSTAT        | Local Status Report Request           | -                       |
+  | 0x65 | ISTAT        | Input Status Report Request           | -                       |
+  | 0x66 | OSTAT        | Output Status Report Request          | -                       |
+  | 0x67 | RSTAT        | Reader Status Report Request          | -                       |
+  | 0x68 | OUT          | Output Control Command                | OutputSettings          |
+  | 0x69 | LED          | Reader Led Control Command            | LedSettings             |
+  | 0x6A | BUZ          | Reader Buzzer Control Command         | BuzzerSettings          |
+  | 0x6B | TEXT         | Text Output Command                   | TextSettings            |
+  | 0x6E | COMSET       | PD Communication Config Command       | ComSettings             |
+  | 0x73 | BIOREAD      | Scan and Send Biometric Data          | Requested Return Format |
+  | 0x74 | BIOMATCH     | Scan and Match Biometric Template     | Biometric Template      |
+  | 0x75 | KEYSET       | Encryption Key Set Command            | EncryptionKey           |
+  | 0x76 | CHLNG        | Challenge/Secure Session Init Request | ChallengeData           |
+  | 0x77 | SCRYPT       | Server Cryptogram                     | EncryptionData          |
+  | 0x7B | ACURXSIZE    | Max ACU receive size                  | Buffer size             |
+  | 0x7C | FILETRANSFER | Send data file to PD                  | File contents           |
+  | 0x80 | MFG          | Manufacturer Specific Command         | Any                     |
+  | 0xA1 | XWR          | Extended write data                   | APDU and details        |
+  | 0xA2 | ABORT        | Abort PD operation                    | -                       |
+  | 0xA3 | PIVDATA      | Get PIV Data                          | Object details          |
+  | 0xA4 | GENAUTH      | Request Authenticate                  | Request details         |
+  | 0xA5 | CRAUTH       | Request Crypto Response               | Challenge details       |
+  | 0xA7 | KEEPACTIVE   | PD read activation                    | Time duration           |
   """
 
   @type t() :: %__MODULE__{
diff --git a/lib/jeff/command/buzzer_settings.ex b/lib/jeff/command/buzzer_settings.ex
index 8e2a50e..90432dc 100644
--- a/lib/jeff/command/buzzer_settings.ex
+++ b/lib/jeff/command/buzzer_settings.ex
@@ -1,14 +1,65 @@
 defmodule Jeff.Command.BuzzerSettings do
+  @moduledoc """
+  Reader Buzzer Control Command
+
+  OSDP v2.2 Specification Reference: 6.11
+  """
+
   defstruct reader: 0x00,
             tone: 0x01,
             on_time: 0x00,
             off_time: 0x00,
             count: 0x00
 
+  @typedoc """
+  Requested tone state
+
+  0x00 = no tone (off) – use of this value is deprecated.
+  0x01 = off
+  0x02 = default tone
+  0x03-0xff = Reserved for future use
+  """
+  @type tone_code() :: 0x00..0xFF
+
+  @typedoc """
+  The ON duration of the sound, in units of 100ms. Must be nonzero unless the
+  tone code is 0x01 (off).
+  """
+  @type on_time() :: 0x00..0xFF
+
+  @typedoc """
+  The OFF duration of the sound, in units of 100ms.
+  """
+  @type off_time() :: 0x00..0xFF
+
+  @typedoc """
+  The number of times to repeat the ON/OFF cycle. 0 = tone continues until
+  another tone command is received.
+  """
+  @type count() :: 0x00..0xFF
+
+  @type t :: %__MODULE__{
+          reader: integer(),
+          tone: tone_code(),
+          on_time: on_time(),
+          off_time: off_time(),
+          count: count()
+        }
+
+  @type param() ::
+          {:reader, integer()}
+          | {:tone, tone_code()}
+          | {:on_time, on_time()}
+          | {:off_time, off_time()}
+          | {:count, count()}
+  @type params :: t() | [param()]
+
+  @spec new(params()) :: t()
   def new(params \\ []) do
     struct(__MODULE__, params)
   end
 
+  @spec encode(params()) :: binary()
   def encode(params) do
     settings = new(params)
 
diff --git a/lib/jeff/command/challenge_data.ex b/lib/jeff/command/challenge_data.ex
index d2bcef2..0b1e600 100644
--- a/lib/jeff/command/challenge_data.ex
+++ b/lib/jeff/command/challenge_data.ex
@@ -1,3 +1,5 @@
 defmodule Jeff.Command.ChallengeData do
+  @moduledoc false
+
   def encode(server_rnd: rnd), do: rnd
 end
diff --git a/lib/jeff/command/com_settings.ex b/lib/jeff/command/com_settings.ex
index edc512a..82d4ac0 100644
--- a/lib/jeff/command/com_settings.ex
+++ b/lib/jeff/command/com_settings.ex
@@ -1,11 +1,31 @@
 defmodule Jeff.Command.ComSettings do
+  @moduledoc """
+  Communication configuration command
+
+  OSDP v2.2 Specification Reference: 6.13
+  """
+
   defstruct address: 0x00,
             baud: 9600
 
+  @type baud() :: 9600
+
+  @type t :: %__MODULE__{
+          address: Jeff.osdp_address(),
+          baud: baud()
+        }
+
+  @type param() ::
+          {:address, Jeff.osdp_address()}
+          | {:baud, baud()}
+  @type params :: t() | [param()]
+
+  @spec new(params()) :: t()
   def new(params \\ []) do
     struct(__MODULE__, params)
   end
 
+  @spec encode(params()) :: binary()
   def encode(params) do
     %{address: address, baud: baud} = new(params)
     <<address, baud::size(4)-unit(8)-little>>
diff --git a/lib/jeff/command/encryption_key.ex b/lib/jeff/command/encryption_key.ex
index edeb1db..bd2cfa8 100644
--- a/lib/jeff/command/encryption_key.ex
+++ b/lib/jeff/command/encryption_key.ex
@@ -1,4 +1,6 @@
 defmodule Jeff.Command.EncryptionKey do
+  @moduledoc false
+
   @secure_channel_base_key 0x01
 
   defstruct [:key, type: @secure_channel_base_key]
diff --git a/lib/jeff/command/encryption_server.ex b/lib/jeff/command/encryption_server.ex
index f05c7c5..c06bb8e 100644
--- a/lib/jeff/command/encryption_server.ex
+++ b/lib/jeff/command/encryption_server.ex
@@ -1,3 +1,4 @@
 defmodule Jeff.Command.EncryptionServer do
+  @moduledoc false
   def encode(cryptogram: cryptogram), do: cryptogram
 end
diff --git a/lib/jeff/command/led_settings.ex b/lib/jeff/command/led_settings.ex
index ed58171..0a91dea 100644
--- a/lib/jeff/command/led_settings.ex
+++ b/lib/jeff/command/led_settings.ex
@@ -1,29 +1,36 @@
 defmodule Jeff.Command.LedSettings do
   @moduledoc """
+  Reader LED control command
+
+  OSDP v2.2 Specification Reference: 6.10
+
   Temporary Control Code Values
-  -----------------------------
-  Code | Description
-  0x00 | NOP – do not alter this LED's temporary settings. The remaining values of the temporary settings record are ignored.
-  0x01 | Cancel any temporary operation and display this LED's permanent state immediately.
-  0x02 | Set the temporary state as given and start timer immediately.
+
+  | Code | Description
+  |------|----------------------------------------------------------------------------------------------------------------------|
+  | 0x00 | NOP – do not alter this LED's temporary settings. The remaining values of the temporary settings record are ignored. |
+  | 0x01 | Cancel any temporary operation and display this LED's permanent state immediately.                                   |
+  | 0x02 | Set the temporary state as given and start timer immediately.                                                        |
 
   Permanent Control Code Values
-  -----------------------------
-  Code | Description
-  0x00 | NOP – do not alter this LED's permanent settings. The remaining values of the temporary settings record are ignored.
-  0x01 | Set the permanent state as given.
+
+  | Code | Description
+  |-----------------------------------------------------------------------------------------------------------------------------|
+  | 0x00 | NOP – do not alter this LED's permanent settings. The remaining values of the temporary settings record are ignored. |
+  | 0x01 | Set the permanent state as given.                                                                                    |
 
   Color Values
-  ------------
-  Value | Description
-  0     | Black (off/unlit)
-  1     | Red
-  2     | Green
-  3     | Amber
-  4     | Blue
-  5     | Magenta
-  6     | Cyan
-  7     | White
+
+  | Value | Description       |
+  |---------------------------|
+  | 0     | Black (off/unlit) |
+  | 1     | Red               |
+  | 2     | Green             |
+  | 3     | Amber             |
+  | 4     | Blue              |
+  | 5     | Magenta           |
+  | 6     | Cyan              |
+  | 7     | White             |
   """
 
   defstruct reader: 0x0,
@@ -40,10 +47,44 @@ defmodule Jeff.Command.LedSettings do
             perm_on_color: 0x00,
             perm_off_color: 0x00
 
+  @type t :: %__MODULE__{
+          reader: integer(),
+          led: integer(),
+          temp_mode: 0x00 | 0x01 | 0x02,
+          temp_on_time: integer(),
+          temp_off_time: integer(),
+          temp_on_color: 0x00..0x07,
+          temp_off_color: 0x00..0x07,
+          temp_timer: integer(),
+          perm_mode: 0x00 | 0x01,
+          perm_on_time: integer(),
+          perm_off_time: integer(),
+          perm_on_color: 0x00..0x07,
+          perm_off_color: 0x00..0x07
+        }
+
+  @type param() ::
+          {:reader, integer()}
+          | {:led, integer()}
+          | {:temp_mode, 0x00 | 0x01 | 0x02}
+          | {:temp_on_time, integer()}
+          | {:temp_off_time, integer()}
+          | {:temp_on_color, 0x00..0x07}
+          | {:temp_off_color, 0x00..0x07}
+          | {:temp_timer, integer()}
+          | {:perm_mode, 0x00 | 0x01}
+          | {:perm_on_time, integer()}
+          | {:perm_off_time, integer()}
+          | {:perm_on_color, 0x00..0x07}
+          | {:perm_off_color, 0x00..0x07}
+  @type params() :: t() | [param()]
+
+  @spec new(params()) :: t()
   def new(params) do
     struct(__MODULE__, params)
   end
 
+  @spec encode(params()) :: binary()
   def encode(params) do
     settings = new(params)
 
diff --git a/lib/jeff/command/output_settings.ex b/lib/jeff/command/output_settings.ex
index 3310921..4bca8bb 100644
--- a/lib/jeff/command/output_settings.ex
+++ b/lib/jeff/command/output_settings.ex
@@ -1,15 +1,20 @@
 defmodule Jeff.Command.OutputSettings do
   @moduledoc """
+  Output control command
+
+  OSDP v2.2 Specification Reference: 6.7
+
   Control Code Values
-  -------------------
-  Code | Description
-  0x00 | NOP – do not alter this output
-  0x01 | Set the permanent state to OFF, abort timed operation (if any)
-  0x02 | Set the permanent state to ON, abort timed operation (if any)
-  0x03 | Set the permanent state to OFF, allow timed operation to complete
-  0x04 | Set the permanent state to ON, allow timed operation to complete
-  0x05 | Set the temporary state to ON, resume permanent state on timeout
-  0x06 | Set the temporary state to OFF, resume permanent state on timeout
+
+  | Code | Description                                                       |
+  |------|-------------------------------------------------------------------|
+  | 0x00 | NOP – do not alter this output                                    |
+  | 0x01 | Set the permanent state to OFF, abort timed operation (if any)    |
+  | 0x02 | Set the permanent state to ON, abort timed operation (if any)     |
+  | 0x03 | Set the permanent state to OFF, allow timed operation to complete |
+  | 0x04 | Set the permanent state to ON, allow timed operation to complete  |
+  | 0x05 | Set the temporary state to ON, resume permanent state on timeout  |
+  | 0x06 | Set the temporary state to OFF, resume permanent state on timeout |
   """
 
   def encode(output: output, code: code, timer: timer) do
diff --git a/lib/jeff/command/text_settings.ex b/lib/jeff/command/text_settings.ex
index eb7f502..f618a1c 100644
--- a/lib/jeff/command/text_settings.ex
+++ b/lib/jeff/command/text_settings.ex
@@ -1,4 +1,6 @@
 defmodule Jeff.Command.TextSettings do
+  @moduledoc false
+
   defstruct reader: 0x00,
             temporary?: false,
             wrap?: false,
diff --git a/lib/jeff/control_info.ex b/lib/jeff/control_info.ex
index 25c9a03..9949009 100644
--- a/lib/jeff/control_info.ex
+++ b/lib/jeff/control_info.ex
@@ -1,4 +1,6 @@
 defmodule Jeff.ControlInfo do
+  @moduledoc false
+
   def encode(%{sequence: seq, check_scheme: cs, security?: sec}) do
     encode(seq, cs, sec)
   end
diff --git a/lib/jeff/device.ex b/lib/jeff/device.ex
index 3b42c46..b7498d5 100644
--- a/lib/jeff/device.ex
+++ b/lib/jeff/device.ex
@@ -1,12 +1,13 @@
 defmodule Jeff.Device do
-  @moduledoc false
+  @moduledoc """
+  Peripheral Device configuration and handling
+  """
 
-  @type device_address :: 0..127
   @type check_scheme :: :checksum | :crc
   @type sequence_number :: 0..3
 
   @type t :: %__MODULE__{
-          address: device_address(),
+          address: Jeff.osdp_address(),
           check_scheme: check_scheme(),
           security?: boolean(),
           secure_channel: term(),
diff --git a/lib/jeff/error_checks.ex b/lib/jeff/error_checks.ex
index 945a08a..4ab175c 100644
--- a/lib/jeff/error_checks.ex
+++ b/lib/jeff/error_checks.ex
@@ -1,4 +1,6 @@
 defmodule Jeff.ErrorChecks do
+  @moduledoc false
+
   use Bitwise
 
   @spec crc(binary()) :: integer()
diff --git a/lib/jeff/events/card_read.ex b/lib/jeff/events/card_read.ex
index ad4abd7..d507d68 100644
--- a/lib/jeff/events/card_read.ex
+++ b/lib/jeff/events/card_read.ex
@@ -4,7 +4,7 @@ defmodule Jeff.Events.CardRead do
   """
 
   @type t :: %__MODULE__{
-          address: 0..127,
+          address: Jeff.osdp_address(),
           data: binary(),
           format: non_neg_integer(),
           length: non_neg_integer(),
diff --git a/lib/jeff/events/keypress.ex b/lib/jeff/events/keypress.ex
index ab79b79..efca10a 100644
--- a/lib/jeff/events/keypress.ex
+++ b/lib/jeff/events/keypress.ex
@@ -4,7 +4,7 @@ defmodule Jeff.Events.Keypress do
   """
 
   @type t :: %__MODULE__{
-          address: 0..127,
+          address: Jeff.osdp_address(),
           count: non_neg_integer(),
           keys: binary(),
           reader: non_neg_integer()
diff --git a/lib/jeff/message.ex b/lib/jeff/message.ex
index 787d722..23115a5 100644
--- a/lib/jeff/message.ex
+++ b/lib/jeff/message.ex
@@ -1,4 +1,6 @@
 defmodule Jeff.Message do
+  @moduledoc false
+
   alias Jeff.{ControlInfo, SecureChannel}
   import Jeff.ErrorChecks
 
diff --git a/lib/jeff/reply.ex b/lib/jeff/reply.ex
index 08df569..5b8e1cf 100644
--- a/lib/jeff/reply.ex
+++ b/lib/jeff/reply.ex
@@ -1,31 +1,34 @@
 defmodule Jeff.Reply do
   @moduledoc """
-  Name      | Code | Description                                  | Data Type
-  ACK       | 0x40 | Command accepted, nothing else to report     | -
-  NAK       | 0x41 | Command not processed                        | ErrorCode
-  PDID      | 0x45 | PD ID Report                                 | IdReport
-  PDCAP     | 0x46 | PD Capabilities Report                       | [Capability]
-  LSTATR    | 0x48 | Local Status Report                          | Report data
-  ISTATR    | 0x49 | Input Status Report                          | Report data
-  OSTATR    | 0x4A | Output Status Report                         | Report data
-  RSTATR    | 0x4B | Reader Status Report                         | Report data
-  RAW       | 0x50 | Reader Data – Raw bit image of card data     | CardData
-  FMT       | 0x51 | Reader Data – Formatted character stream     | Card data
-  KEYPAD    | 0x53 | Keypad Data                                  | KeypadData
-  COM       | 0x54 | PD Communications Configuration Report       | ComData
-  BIOREADR  | 0x57 | Biometric Data                               | Biometric data
-  BIOMATCHR | 0x58 | Biometric Match Result                       | Result
-  CCRYPT    | 0x76 | Client's ID, Random Number, and Cryptogram   | EncryptionClient
-  BUSY      | 0x79 | PD is Busy reply                             | -
-  RMAC_I    | 0x78 | Initial R-MAC                                | Encryption Data
-  FTSTAT    | 0x7A | File transfer status                         | Status details
-  PIVDATAR  | 0x80 | PIV Data Reply                               | credential data
-  GENAUTHR  | 0x81 | Authentication response                      | response details
-  CRAUTHR   | 0x82 | Response to challenge                        | response details
-  MFGSTATR  | 0x83 | MFG specific status                          | status details
-  MFGERRR   | 0x84 | MFG specific error                           | error details
-  MFGREP    | 0x90 | Manufacturer Specific Reply                  | Any
-  XRD       | 0xB1 | Extended Read Response                       | APDU and details
+  Replies are sent from a PD to an ACU in response to a command
+
+  | Name      | Code | Description                                | Data Type        |
+  |-----------|------|--------------------------------------------|------------------|
+  | ACK       | 0x40 | Command accepted, nothing else to report   | -                |
+  | NAK       | 0x41 | Command not processed                      | ErrorCode        |
+  | PDID      | 0x45 | PD ID Report                               | IdReport         |
+  | PDCAP     | 0x46 | PD Capabilities Report                     | [Capability]     |
+  | LSTATR    | 0x48 | Local Status Report                        | Report data      |
+  | ISTATR    | 0x49 | Input Status Report                        | Report data      |
+  | OSTATR    | 0x4A | Output Status Report                       | Report data      |
+  | RSTATR    | 0x4B | Reader Status Report                       | Report data      |
+  | RAW       | 0x50 | Reader Data – Raw bit image of card data   | CardData         |
+  | FMT       | 0x51 | Reader Data – Formatted character stream   | CardData         |
+  | KEYPAD    | 0x53 | Keypad Data                                | KeypadData       |
+  | COM       | 0x54 | PD Communications Configuration Report     | ComData          |
+  | BIOREADR  | 0x57 | Biometric Data                             | Biometric data   |
+  | BIOMATCHR | 0x58 | Biometric Match Result                     | Result           |
+  | CCRYPT    | 0x76 | Client's ID, Random Number, and Cryptogram | EncryptionClient |
+  | BUSY      | 0x79 | PD is Busy reply                           | -                |
+  | RMAC_I    | 0x78 | Initial R-MAC                              | Encryption Data  |
+  | FTSTAT    | 0x7A | File transfer status                       | Status details   |
+  | PIVDATAR  | 0x80 | PIV Data Reply                             | credential data  |
+  | GENAUTHR  | 0x81 | Authentication response                    | response details |
+  | CRAUTHR   | 0x82 | Response to challenge                      | response details |
+  | MFGSTATR  | 0x83 | MFG specific status                        | status details   |
+  | MFGERRR   | 0x84 | MFG specific error                         | error details    |
+  | MFGREP    | 0x90 | Manufacturer Specific Reply                | Any              |
+  | XRD       | 0xB1 | Extended Read Response                     | APDU and details |
   """
 
   use Bitwise
@@ -44,7 +47,7 @@ defmodule Jeff.Reply do
   @type t() :: %__MODULE__{
           address: byte(),
           code: byte(),
-          data: binary() | map(),
+          data: binary() | map() | list(),
           name: atom()
         }
 
@@ -108,9 +111,7 @@ defmodule Jeff.Reply do
     address &&& 0b01111111
   end
 
-  defp decode(_name, nil), do: nil
-  defp decode(_name, <<>>), do: nil
-  defp decode(ACK, _data), do: nil
+  defp decode(ACK, _data), do: Jeff.Reply.ACK
   defp decode(NAK, data), do: ErrorCode.decode(data)
   defp decode(PDID, data), do: IdReport.decode(data)
   defp decode(PDCAP, data), do: Capability.decode(data)
@@ -120,6 +121,8 @@ defmodule Jeff.Reply do
   defp decode(RAW, data), do: CardData.decode(data)
   defp decode(CCRYPT, data), do: EncryptionClient.decode(data)
   defp decode(RMAC_I, data), do: data
+  defp decode(_name, nil), do: nil
+  defp decode(_name, <<>>), do: nil
   defp decode(name, data), do: Module.concat(__MODULE__, name).decode(data)
 
   def code(name), do: @codes[name]
diff --git a/lib/jeff/reply/capability.ex b/lib/jeff/reply/capability.ex
index 2818176..b385023 100644
--- a/lib/jeff/reply/capability.ex
+++ b/lib/jeff/reply/capability.ex
@@ -1,6 +1,19 @@
 defmodule Jeff.Reply.Capability do
+  @moduledoc """
+  Peripheral Device Capabilities Report
+
+  OSDP v2.2 Specification Reference: 7.5
+  """
+
   defstruct [:function, :compliance, :number_of, :description]
 
+  @type t :: %__MODULE__{
+          function: integer(),
+          compliance: integer(),
+          number_of: integer(),
+          description: String.t()
+        }
+
   @functions %{
     1 => "Contact Status Monitoring",
     2 => "Output Control",
@@ -29,6 +42,7 @@ defmodule Jeff.Reply.Capability do
     }
   end
 
+  @spec decode(binary()) :: [t()]
   def decode(data) do
     do_decode(data, [])
   end
diff --git a/lib/jeff/reply/card_data.ex b/lib/jeff/reply/card_data.ex
index acfbcb7..3632d03 100644
--- a/lib/jeff/reply/card_data.ex
+++ b/lib/jeff/reply/card_data.ex
@@ -1,4 +1,10 @@
 defmodule Jeff.Reply.CardData do
+  @moduledoc """
+  Card Data Report
+
+  OSDP v2.2 Specification Reference: 7.10
+  """
+
   defstruct [:reader, :format, :length, :data]
 
   def decode(<<reader, format, length::size(16)-little, data::binary>>) do
diff --git a/lib/jeff/reply/com_data.ex b/lib/jeff/reply/com_data.ex
index 6aeac38..f1cc0bb 100644
--- a/lib/jeff/reply/com_data.ex
+++ b/lib/jeff/reply/com_data.ex
@@ -1,7 +1,19 @@
 defmodule Jeff.Reply.ComData do
+  @moduledoc """
+  Communication Configuration Report
+
+  OSDP v2.2 Specification Reference: 7.13
+  """
+
   defstruct address: 0x00,
             baud: 9600
 
+  @type t :: %__MODULE__{
+          address: Jeff.osdp_address(),
+          baud: 9600
+        }
+
+  @spec decode(binary()) :: t()
   def decode(<<address, baud::size(4)-unit(8)-little>>) do
     %__MODULE__{address: address, baud: baud}
   end
diff --git a/lib/jeff/reply/encryption_client.ex b/lib/jeff/reply/encryption_client.ex
index d2d25a9..2cdf913 100644
--- a/lib/jeff/reply/encryption_client.ex
+++ b/lib/jeff/reply/encryption_client.ex
@@ -1,4 +1,6 @@
 defmodule Jeff.Reply.EncryptionClient do
+  @moduledoc false
+
   defstruct [:cuid, :rnd, :cryptogram]
 
   def decode(<<
diff --git a/lib/jeff/reply/error_code.ex b/lib/jeff/reply/error_code.ex
index 28d40b1..025368d 100644
--- a/lib/jeff/reply/error_code.ex
+++ b/lib/jeff/reply/error_code.ex
@@ -1,6 +1,17 @@
 defmodule Jeff.Reply.ErrorCode do
+  @moduledoc """
+  Negative Acknowledge – Error Response
+
+  OSDP v2.2 Specification Reference: 7.3
+  """
+
   defstruct [:code, :description]
 
+  @type t :: %__MODULE__{
+          code: integer(),
+          description: String.t()
+        }
+
   @description %{
     0x00 => "No error",
     0x01 => "Message check character(s) error (bad cksum/crc)",
diff --git a/lib/jeff/reply/id_report.ex b/lib/jeff/reply/id_report.ex
index beccb09..c2cf6bb 100644
--- a/lib/jeff/reply/id_report.ex
+++ b/lib/jeff/reply/id_report.ex
@@ -1,6 +1,20 @@
 defmodule Jeff.Reply.IdReport do
+  @moduledoc """
+  Device identification Report
+
+  OSDP v2.2 Specification Reference: 7.4
+  """
+
   defstruct [:vendor, :model, :version, :serial, :firmware]
 
+  @type t :: %__MODULE__{
+          firmware: String.t(),
+          model: integer(),
+          serial: String.t(),
+          vendor: String.t(),
+          version: integer()
+        }
+
   def decode(data) do
     <<
       vendor1,
diff --git a/lib/jeff/reply/keypad_data.ex b/lib/jeff/reply/keypad_data.ex
index 79522b6..97fe40f 100644
--- a/lib/jeff/reply/keypad_data.ex
+++ b/lib/jeff/reply/keypad_data.ex
@@ -1,4 +1,10 @@
 defmodule Jeff.Reply.KeypadData do
+  @moduledoc """
+  Keypad Data Report
+
+  OSDP v2.2 Specification Reference: 7.12
+  """
+
   defstruct [:reader, :count, :keys]
 
   def decode(<<reader, count, keys::binary>>) do
diff --git a/lib/jeff/reply/local_status.ex b/lib/jeff/reply/local_status.ex
index 2f94fca..26f0bc8 100644
--- a/lib/jeff/reply/local_status.ex
+++ b/lib/jeff/reply/local_status.ex
@@ -1,6 +1,23 @@
 defmodule Jeff.Reply.LocalStatus do
+  @moduledoc """
+  Local Status Report
+
+  OSDP v2.2 Specification Reference: 7.6
+  """
+
   defstruct [:tamper, :power]
 
+  @type tamper_status() :: :normal | :tamper
+  @type power_status() :: :normal | :failure
+  @type tamper_code() :: 0x00 | 0x01
+  @type power_code() :: 0x00 | 0x01
+
+  @type t :: %__MODULE__{
+          tamper: tamper_status(),
+          power: power_status()
+        }
+
+  @spec new(tamper_code(), power_code()) :: t()
   def new(tamper_code, power_code) do
     %__MODULE__{
       tamper: tamper_status(tamper_code),
@@ -8,6 +25,7 @@ defmodule Jeff.Reply.LocalStatus do
     }
   end
 
+  @spec decode(binary()) :: t()
   def decode(<<tamper, power>>) do
     __MODULE__.new(tamper, power)
   end
diff --git a/lib/jeff/secure_channel.ex b/lib/jeff/secure_channel.ex
index c5df302..e8538b0 100644
--- a/lib/jeff/secure_channel.ex
+++ b/lib/jeff/secure_channel.ex
@@ -1,4 +1,6 @@
 defmodule Jeff.SecureChannel do
+  @moduledoc false
+
   use Bitwise
 
   defstruct [
diff --git a/test/bus_test.exs b/test/bus_test.exs
index a63a2bb..aa7c0a7 100644
--- a/test/bus_test.exs
+++ b/test/bus_test.exs
@@ -141,7 +141,7 @@ defmodule BusTest do
     poll_command = Command.new(0x01, POLL)
     ack_reply = Reply.new(0x01, ACK)
     id_command = Command.new(0x01, ID)
-    pdid_reply = Reply.new(0x01, PDID)
+    pdid_reply = Reply.new(0x01, PDID_TEST)
 
     bus = Bus.new()
     bus = Bus.add_device(bus, address: 0x01)
diff --git a/test/reply_test.exs b/test/reply_test.exs
index 268677e..96a6ac3 100644
--- a/test/reply_test.exs
+++ b/test/reply_test.exs
@@ -5,12 +5,12 @@ defmodule ReplyTest do
 
   test "build a new reply from a message" do
     reply = Reply.new(%Message{address: 0x01, code: 0x40, data: nil})
-    assert %{code: 0x40, name: ACK, data: nil} = reply
+    assert %{code: 0x40, name: ACK, data: Jeff.Reply.ACK} = reply
   end
 
   test "build a new reply from module" do
     reply = Reply.new(0x01, ACK)
-    assert %{address: 0x01, code: 0x40, name: ACK, data: nil} = reply
+    assert %{address: 0x01, code: 0x40, name: ACK, data: Jeff.Reply.ACK} = reply
   end
 
   test "build a reply with an unknown code" do