Try to get this working on all machines

[SukkoPera]

AmiBay user eslapion indicates two possible modifications to make this work on all C16's:

1. The first one is to put a switch on pin 1 (/CE) of the two 4416 of the C16, toggling between ground and 5Vdc - by default they are grounded.

2. The second one is to use a modified 48 pin socket under the TED which will disconnect CAS from the mainboard and have it on a separate wire going to the 64k RAM cartridge. Just put a pull-up resistor on the 64k RAM cartridge which pulls-up the CAS line of the C16 board.

My opinion:
Solution 1 is well-known but it's not really easy to do, as it requires desoldering the whole chips to get a "clean job".

Solution 2 is more interesting, but I find this would be ok if we wanted to make a board to be sandwiched between the TED and socket, carrying the RAM chips and whatever is needed. An internal expansion that cannot be removed, I mean. This is because the machine would no longer work if the expansion is unplugged at that point (unless you also bring out the /CAS signal going to the chips, add a switch and whatever).

The modification I am looking for would ideally be to add the missing circuitry so that the machine will use the expansion correctly, but so that it will still work correctly if the expansion is unplugged, without any manual intervention. Ideally, this mod would also be easy to install and it shouldn't require to cut any tracks. But I guess the latter is impossible to achieve, as the whole thing implies the splitting of the /RAS (or /CAS) signal in two, one addressing the internal chips and one going to the cartridge slot. Equations are easy:

!ras_int = !ras & (!a14 & !a15) => ras_int = ras | !(!a14 & !a15) => ras_int = ras | a14 | a15
!ras_ext = !ras & (a14 | a15) => ras_ext = ras | !(a14 | a15) => ras_ext = ras | (!a14 & !a15)

ras	a14	a15	ras_int	ras_ext
0	0	0	0	1
0	0	1	1	0
0	1	0	1	0
0	1	1	1	0
1	0	0	1	1
1	0	1	1	1
1	1	0	1	1
1	1	1	1	1

The point here is to rewrite these in order to use the least possible logic gates and to make sure that the memory detection routine in the ROM always does the right thing.

I think it can be done with a single 74253 or 4052 (Dual 4-TO-1 Multiplexers):

Pin	74253	CD4052B
1	/RAS	GND
2	a15	Vcc
3	Vcc	ras_int (with pull-up to Vcc)
4	Vcc	Vcc
5	Vcc	Vcc
6	GND	/RAS
7	ras_int (with pull-up to Vcc)	GND (power)
8	GND (power)	GND (power)
9	ras_ext (with pull-up to Vcc)	a15
10	Vcc	a14
11	GND	GND
12	GND	Vcc
13	GND	ras_ext (with pull-up to Vcc)
14	a14	GND
15	/RAS	GND
16	Vcc (power)	Vcc (power)

Another possibility is here.