Replacing VIC-20 Rom char (901460-03) with 2732 - unespected behavior!

[giobbi]

Hi guys! Nice to be here again, since at least I found some time to solve some new problems I hadn't LOL

First of all, I hope all of you are healthy, in good shape and covid-free!!

Here's the problem:

I found a broken Char ROM (labeled 901460-03) in one of my VIC-20. So I built a 2732 adapter for a right replacement.
I used this schematic, I suppose I got from the Ray Carlsen's site. I didn't find any other clue about a 2332 to 2732 adapter, though...

BTW this adapter works fine for PET ROMs replacement, I already built few of them...



But something went wrong.
This is the result:



and, very strange, I got the same result plugging directly the 2732 into the socket, without the adapter:


(you can see I'm carrying the adapter).

This confused me a lot.

Should I use some diodes like I used to do in other adapters? Or, do the VIC-20 ROM Char has some different pinout?

 

[giobbi]

In a long thread elsewhere, I found the right clue.

The adapter I was using is totally wrong for the VIC-20 (but good for PET ROM replacements).

The only change you need to do is to cross the 18 and 21 pins between Eprom and VIC-20 socket. So the adapter is simpler than a PET adapter.


Taken from the other thread:

To replace the 2332 PROM with a properly wired 2732 EPROM you need to wire:
- CE2(21) in the socket to /OE(18) in the 2732 EPROM and A11(18) to A11(21)

 

[Steph]

I believe the 2332 prom is 100% pin compatible with the 2532 eprom.

The 2532 is a strange beast created by Texas Instrument (often called TMS-2532)

Not all programmers support this , but many do.
( Look under the TI device list to see if you can find it )

Good luck.

Steph

 

[Steph]

Re-reading this thread, I see that the 2332 has two chip/output selects.
Pin 20 : CE1 must be LOW
Pin 21 : CE2 must be HIGH

On the 2532 eproms BOTH of these must be low.

So while the 2532 eproms has the exact same pinout at the 2332 prom, CE2 is set differently,
so you need to tie pin 21 on the 2532 eprom to low.

Stephan

 

[giobbi]

Re-reading this thread, I see that the 2332 has two chip/output selects.
Pin 20 : CE1 must be LOW
Pin 21 : CE2 must be HIGH

On the 2532 eproms BOTH of these must be low.

So while the 2532 eproms has the exact same pinout at the 2332 prom, CE2 is set differently,
so you need to tie pin 21 on the 2532 eprom to low.

Stephan

Nowadays, 2732 is an easier replacement and most eprom programmers work fine with it. 2532 are usually more expensive.

I already used a lot the 2732 eproms as replacement for PET rom using an adapter; I just discovered the VIC-20 character ROM needed another kind of adapter.

Just swap PINs 18 and 21 between eprom and socket (18 to 21 and 21 to 18 ) and you're done!

 

[Steph]

Nowadays, 2732 is an easier replacement and most eprom programmers work fine with it. 2532 are usually more expensive.

I already used a lot the 2732 eproms as replacement for PET rom using an adapter; I just discovered the VIC-20 character ROM needed another kind of adapter.

Just swap PINs 18 and 21 between eprom and socket (18 to 21 and 21 to 18 ) and you're done!

View attachment 67207

Your diagram is interesting...

You have Pin 18 on the 2732 (CE!) going to pin 21 on the 2332 (CS2)
(so you have a LOW signal CE! going to a pin expecting a HIGH signal [CS2] ).

If that actually works, then CS2 on the 2332 is actually active low (CS2!).
That is possible since CS1 and CS2 on the 2332 are set during manufacturing.
So perhaps both are set LOW.

If this is the case, then a 2532 eprom is a DIRECT replacement, ( no adapter needed )

If your programmer support this ( TI TMS-2532 ) then you should be able to read it directly.
(... and get the same result as reading it as a 2732 with your adapter ).

It's also possible that the 2332 used on the PET has a different set of CS than the VIC
which is why you would need a different adapter for each.

Steph

 

[iz8dwf]

The VIC-20 chargen ROM is mask programmed with both /CS1 and /CS2 (i.e. both active low). PET's 2332 instead have opposite polarities on the two chip select lines.
A 2532 EPROM has one single select line active low, it works on non-universal boards PET as a direct replacement. On Universal board (CRTC based) PETs, CS1 is used as a NOROM signal from the 6502 socket to allow add-ons to disable onboard ROMs.
A 2532 EPROM won't then work as VIC-20 chargen because of the need for two active low selects.
A 2732 EPROM has two active low signals (actually one select and one output enable, but that works fine too as select) so it can be adapted for the VIC-20 chargen and for non-CRTC PETs.

HTH
Frank IZ8DWF

 

[MikeS]

As usual, a clear and concise explanation of a perpetually confusing issue!

BTW, the disk drives also use some ROMs with active high selects; Jim B's adapters provide various configuration options:
file:///D:/My%20Documents/Temp/Commodore%20-%20Hardware%20-%20Adapters%20-%20RETRO%20Innovations.html

 

[giobbi]

A 2732 EPROM has two active low signals (actually one select and one output enable, but that works fine too as select) so it can be adapted for the VIC-20 chargen and for non-CRTC PETs.

A little bit confused about this part (sorry)...

Do you mean the same adapter that works for VIC-20 (just crossing pin 18 and 21) can be used as adapter for non-CRTC PETs?

 

[iz8dwf]

I wrote that it can be adapted, I've never wrote that it can be done with the same adapter. I've also written that PET's 2332 ROMs have different select polarities than the VIC-20 chargen one.
So in the case of non-CRTC PETs you don't connect CS2 (pin 21) to the 2732.

Frank

 

[giobbi]

I wrote that it can be adapted, I've never wrote that it can be done with the same adapter. I've also written that PET's 2332 ROMs have different select polarities than the VIC-20 chargen one.
So in the case of non-CRTC PETs you don't connect CS2 (pin 21) to the 2732.

Frank

I didn't say you wrote that, LOL, I was simply confused since I don't understand much about that
I just asked.

 

[iz8dwf]

As usual, a clear and concise explanation of a perpetually confusing issue!

BTW, the disk drives also use some ROMs with active high selects; Jim B's adapters provide various configuration options:
file:///D:/My%20Documents/Temp/Commodore%20-%20Hardware%20-%20Adapters%20-%20RETRO%20Innovations.html

Hi Mike!
Yes, the old IEEE drives also use 2332 ROMs with two opposite-polarity selects, and they're both needed. So yes, in that case a 2532 or 2732 without any active adapter wouldn't work.
I have also made my version of an universal ROM adapter (from the 2316 to the 2364). I can share the files if someone is interested, but I ended using a SO-28 28C64 as the memory chip,
so that's not something everyone can program easily. However, the EEPROM can also be programmed once soldered to the adapter by connecting the provided /WR signal.
I know, not so "user friendly" but it works for me

Frank