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serial protocol.txt
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;***************************************************************************************
;
; FDC+ Serial Drive Communications Protocol
;
; Version Date Author Comments
; 1.0 3/05/15 M. Douglas Original version
;
;-------------------------------------------------------------------------------------
;
; Communication with the server is over a serial port at 403.2K Baud, 8N1.
; All transactions are initiated by the FDC. The second choice for baud rate
; is 460.8K. Finally, 230.4K is the most likely supported baud rate on the PC
; if 403.2K and 460.8K aren't avaialable.
;
; 403.2K is the preferred rate as it allows full-speed operation and is the
; most accurate of the three baud rate choices on the FDC. 460.8K also allows
; full speed operation, but the baud rate is off by about 3.5%. This works, but
; is borderline. 230.4K is available on most all serial ports, is within 2% of
; the FDC baud rate, but runs at 80%-90% of real disk speed.
;
; FDC TO SERVER COMMANDS
; Commands from the FDC to the server are fixed length, ten byte messages. The
; first four bytes are a command in ASCII, the remaining six bytes are grouped
; as three 16 bit words (little endian). The checksum is the 16 bit sum of the
; first eight bytes of the message.
TODO: use a unsigned 16bit
;
; Bytes 0-3 Bytes 4-5 as Word Bytes 6-7 as Word Bytes 8-9 as Word
; --------- ----------------- ----------------- -----------------
; Command Parameter 1 Parameter 2 Checksum
;
; Commands:
; STAT - Provide and request drive status. The FDC sends the selected drive
; number and head load status in Parameter 1 and the current track
; number in Parameter 2. The Server responds with drive mount status
; (see below). The LSB of Parameter 1 contains the currently selected
; drive number or 0xff is no drive is selected. The MSB of parameter 1
; is zero if the head is loaded, non-zero if not loaded.
;
; The FDC issues the STAT command about ten times per second so that
; head status and track number information is updated quickly. The
; server may also want to assume the drive is selected, the head is
; loaded, and update the track number whenever a READ is received.
;
; READ - Read specified track. Parameter 1 contains the drive number in the
; MSNibble. The lower 12 bits contain the track number. Transfer length
; length is in Parameter 2 and must be the track length. Also see
; "Transfer of Track Data" below.
;
; WRIT - Write specified track. Parameter 1 contains the drive number in the
; MSNibble. The lower 12 bits contain the track number. Transfer length
; must be track length. Server responds with WRIT response when ready
; for the FDC to send the track of data. See "Transfer of Track Data" below.
;
;
; SERVER TO FDC
; Reponses from the server to the FDC are fixed length, ten byte messages. The
; first four bytes are a response command in ASCII, the remaining six bytes are
; grouped as three 16 bit words (little endian). The checksum is the 16 bit sum
; of the first eight bytes of the message.
;
; Bytes 0-3 Bytes 4-5 as Word Bytes 6-7 as Word Bytes 8-9 as Word
; --------- ----------------- ----------------- -----------------
; Command Response Code Reponse Data Checksum
;
; Commands:
; STAT - Returns drive status in Response Data with one bit per drive. "1" means a
; drive image is mounted, "0" means not mounted. Bits 15-0 correspond to
; drive numbers 15-0. Response code is ignored by the FDC.
;
; WRIT - Issued in repsonse to a WRIT command from the FDC. This response is
; used to tell the FDC that the server is ready to accept continuous transfer
; of a full track of data (response code word set to "OK." If the request
; can't be fulfilled (e.g., specified drive not mounted), the reponse code
; is set to NOT READY. The Response Data word is don't care.
TODO: Should checksum be sent with WRIT ack?
;
; WSTA - Final status of the write command after receiving the track data is returned
; in the repsonse code field. The Response Data word is don't care.
;
; Reponse Code:
; 0x0000 - OK
; 0x0001 - Not Ready (e.g., write request to unmounted drive)
; 0x0002 - Checksum error (e.g., on the block of write data)
; 0x0003 - Write error (e.g., write to disk failed)
;
;
; TRANSFER OF TRACK DATA
; Track data is sent as a sequence of bytes followed by a 16 bit, little endian
; checksum. Note the Transfer Length field does NOT include the two bytes of
; the checksum. The following notes apply to both the FDC and the server.
;
; ERROR RECOVERY
; The FDC uses a timeout of one second after the last byte of a message or data block
; is sent to determine if a transmission was ignored.
;
; The server should ignore commands with an invalid checksum. The FDC may retry the
; command if no response is received. An invalid checksum on a block of write
; data should not be ignored, instead, the WRIT response should have the
; Reponse Code field set to 0x002, checksum error.
;
; The FDC ignores responses with an invalid checksum. The FDC may retry the command
; that generated the response by sending the command again.
;
;***************************************************************************************