I seem to recall that 2716 char EPROMS worked on some revisions of the ][/+ mainboards. Which revisions support that? I do not recall offhand what was changed in the char ROM pimnouts.
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Can't help with the board revisions. But what changed is pin 21. EPROMs lack the 3rd chip select signal. For EPROMs pin 21 is reserved for Vpp (programming voltage). For the ROMs that's "/CS3" instead. Board designs which use all three chipselects, can only work with ROMs. Designs which simply connect pin 21 to ground, work with ROMs just as well as with EPROMs. You can use a continuity tester to check if pin 21 is fixed to ground (and thus also EPROMs are supported).
The answer is "almost all of them".
Most of the II+ systems you'll see are Rev-7 and RFI revision motherboards, and those are most common by far.
Even if the board you end up with can't support a 2716 in the CG position it's a doddle to adapt.
As pointed out by baldrick, the Rev 7 (part number 820-0001-07) and the RFI (part numbers 820-0044-*) allow 2716 to take place of the 2316B Characters ROM.
It is still possible to find Rev 4 (and previous ones) motherboards. They are easy to spot, thanks to the jumper memory blocks on the left side of the board.
I have Rev 7, RFI, and Rev 4 boards. I thought that one of them (perhaps Rev 4 and earlier?) could not use the 2716 without an adaptation, so if that is correct, then at least I was not remembering wrongly.
Well, the Beltron char ROM is either scrambled, or not compatible. I tried it on a ][+ Rev 7, and it displayed corrupted characters. They were clearly close to what was intended, but also clearly gibberish, so IDK if it requires the Beltron logic design, the Beltron ROMs (I have yet to determine if these are good), or something else.
I was going to dump it, if it worked, but as yet I do not know, and it is back on the Beltron board, which alas, does not work, and is on my bench for logic probing. I'm hoping that the fact that the original PSU output two +5VDC (and the board has sockets for both), but only one Earth; versus the ][+ PSU with two Earths and one +5VDC, is not the culprit. I am using a stock ][+ PSU and I am only wired to one +5, and the second Earth output is floating.
As far as I can tell, on the Beltron both +5s are tied together.
Have you got a 2716 ROM .bin file for a II+ character generator?
Burn one and see if your II+ can handle it (it ought to).
If I had to guess I'd say that your Beltron CG ROM might be corrupt...
I have not done a dump of the EPROM, but the Beltron board itself does not do anything, no beep, no video, no card access, so it is in my 'problem' bin.
Until I sort out its issues, the status of its CHAR ROM is immaterial. I subbed out a ][+ mainboard for its mainboard and the system is working, but I need to r/r one keyswitch (numpad '7') to get it fully operable. All of the special key functions work w/o the Beltron CHAR ROM, so that's not an issue. It took me a while to figure out that I needed to press the power key + normal key to get the desired effect--to be frank, I expected the special keys to operate off of control or some control combo, but on inspecting the design, the power key is wired to the matrix, and holding it wil the desired key produces the right result on a stock ][+ board.
I haven't used many weird clone systems, so the power key having a function of any sort took me a bit by shock, but that's just life. I thought that the chr ROM was needed to produce events such as shortcuts for CATALOG, but no, this is all handled by the KB itself, and I do need to dump that ROM.