24pin to 28 pin socket adapters for C64/1541 advice

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eslapion
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Re: 24pin to 28 pin socket adapters for C64/1541 advice

Post by eslapion »

BTW, for those who DON'T want to have to use an adapter, I just got myself a nice pile of MCM68764 EPROMs which are pin compatible with 2364 ROMs.


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Borat
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Re: 24pin to 28 pin socket adapters for C64/1541 advice

Post by Borat »

Sorry for the necro-post but this is the most appropriate thread.

@eslapion I bought a batch of your adapter boards many years ago but have never used them.

Image

Just recently I wanted to replace a C64 character ROM with a 27128 so I got one of my boards. I have a quick question here: why is pin 21 (CE2) of the 2332 linked to pin 2 (A12) of the 27128? Looking at the C64 schematic CE2 is at 5V so this means A12 is also at 5V. Should A12 not be at GND as it's not needed? Or is it a case that unused address pins should be high?

Thanks!
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Re: 24pin to 28 pin socket adapters for C64/1541 advice

Post by eslapion »

@Borat

This adapter is really designed to adapt 27(C)64 or 128 or 256 or 512 (E)PROMs to a 2364 ROM socket.

On the 2364, pin 21 is A12 and so I attached it to ... A12.

It's kinda funny, isn't it? I attached A12 to A12... :lol:

A 2332 being half the capacity of a 2364, it has no need for this last address line. The people at JEDEC decided to assign it to an extra chip enable which is active high (on the C64) to help with address decoding but AFAIK, it isn't used in the C64, it's just tied to 5V.

The funny thing is the extra CE actually is used in the VIC-20 and it's active low on that machine so character generators of one machine will not work on the other (VIC vs 64).
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Re: 24pin to 28 pin socket adapters for C64/1541 advice

Post by Borat »

So could I use it for the C64 char rom if I cut the trace and grounded A12?

Am I right in assuming unused address lines like A12 or A13 should be grounded and not at 5V?

Thanks.
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eslapion
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Re: 24pin to 28 pin socket adapters for C64/1541 advice

Post by eslapion »

Borat wrote:So could I use it for the C64 char rom if I cut the trace and grounded A12?

Am I right in assuming unused address lines like A12 or A13 should be grounded and not at 5V?

Thanks.
It depends.

If you absolutely want to program the lower half then you'd need to have that specific pin grounded.

IMHO, it's a lot simpler to use a 27(C)64 and program the character data in the upper half and leave A12/CE2 as it is.

Also, there are two solder pads on that adapter board that can be used for selecting which area of an EPROM is visible to the computer. Pin 27 in particular is !P (program active low) which you certainly don't want grounded. Pin 26 is NC on a 2764 but it becomes A13 on a 27128.

You indicated you wanted to use a 27128 to start with so the easiest way to get to the desired result with no extra component or wire is to program the character data in the upper last 4kBytes of your 27128 and solder both pads to Vcc.
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