Quicky: PET Character ROM replacement

[Eudimorphodon]

I'm sure this has been asked before (probably even by me), but my search-foo is failing along with my memory and I sort of need to know quickly: Does a 2716 make an acceptable EPROM substitute for the PET character generator ROM? I *thought* the answer was yes, but I'm sure someone knows for sure off the top of their head.

 

[carlsson]

As far as I know, the answer is yes. Almost all of my PETs have TMS2516 or 2716 EPROMs for character ROM, since they're equipped with a Swedish character SET (ÅÄÖ in case you forgot).

 

[MikeS]

Yes, except for the two versions of the 2001 that used 6540 ROMs; I think some of the vary late models (B, P?) also had provisions for an alternate character set.

Always curious: why do you need to know?

 

[scouter3d]

Hi, in my 2001-8 a 2716 only fits with the help of an adapter...

 

[MikeS]

Hi, in my 2001-8 a 2716 only fits with the help of an adapter...

Well, like I said there are two versions of the static-RAM 2001 that used 6540 ROMs which would indeed require an adapter; I assume that your other ROMs have numbers like MPS6540 and/or 901447 or 901465 on them?

 

[Eudimorphodon]

Yes, except for the two versions of the 2001 that used 6540 ROMs; I think some of the vary late models (B, P?) also had provisions for an alternate character set.

Yeah *foreheadslap*, I realize now I probably should of specified *which* model. It's for a Dynamic Board. (2001-N->9" 4032, the bog-standard "non-calculator" guts.) Sounds like a 2716 should do it, then.

Always curious: why do you need to know?

No particularly exotic reason, I just have a chance to get some EPROMs burned. (If I didn't get my order in quick it'd probably be a few weeks before I got another chance.) I'd like to get back to finishing the repair of the sad-sack motherboard that came in my 9" 4032, which I replaced with a board I got in a trade after getting it to a "boots to BASIC with full RAM but still has video corruption and IEEE-488 issues" state. The replacement board was missing a character generator so I'm one short unless I want to borrow one while I'm troubleshooting... and pulling and replacing those fragile old ROM chips isn't my idea of a good time.

Granted I've had like *no* time lately to work on anything, but... yay.

 

[carlsson]

For a year or two, I've played with the thought of burning a custom font to install on a PET. Perhaps even a graphics ROM switcher for any programs/games requiring custom graphics. I've read about something similar, a hires board for PET but I can't remember if it implemented it by replacing character ROM with RAM and fake bitmapping like how one would do it on e.g. VIC-20.

 

[sjgray]

For a year or two, I've played with the thought of burning a custom font to install on a PET. Perhaps even a graphics ROM switcher for any programs/games requiring custom graphics. I've read about something similar, a hires board for PET but I can't remember if it implemented it by replacing character ROM with RAM and fake bitmapping like how one would do it on e.g. VIC-20.

This is the only one I am aware of:

http://home.germany.net/nils.eilers/chargen/index.htm

I'm not an electronics expert but I don't think dual-port ram is really required. AFAIK due to the video chip and cpu accessing ram on alternate clock phases there should be no conflicts updating soft-character-set ram. Perhaps there is some other reason. Anyway, this looks like an interesting project.

Steve

 

[for()]

You're right: a dual-port RAM is not required. But I'd like to explain, why it's a big advantage to use one anyway: if you'd use a standard single-port RAM, you'd have to mux the CPU-data-address bus and the CRTC-data-address bus to the RAM. This would require a lot of 74LS157 or 74LS244, let's say 6(!) 74LS244.

The CHARGEN at my website is somewhat outdated. I wouldn't use this esoteric PHI-2-delay with the resistor/capacitor any longer and I'd use a single dual-port RAM IDT7132 (4 KB, PLCC) instead of two huge 1KB RAMs.

 

[sjgray]

You're right: a dual-port RAM is not required. But I'd like to explain, why it's a big advantage to use one anyway: if you'd use a standard single-port RAM, you'd have to mux the CPU-data-address bus and the CRTC-data-address bus to the RAM. This would require a lot of 74LS157 or 74LS244, let's say 6(!) 74LS244.

The CHARGEN at my website is somewhat outdated. I wouldn't use this esoteric PHI-2-delay with the resistor/capacitor any longer and I'd use a single dual-port RAM IDT7132 (4 KB, PLCC) instead of two huge 1KB RAMs.

Thanks for the explanation! I once looked for dual-port ram but couldn't find anything large and cheap. Do you know of any in the 32-64K area? I guess the other option is to use a CPLD chip with internal ram programmed to behave like dual-port ram.

Do you have any plans on updating your project? Perhaps if there was enough interest you could take pre-orders for a small run?

Steve

 

[for()]

I once looked for dual-port ram but couldn't find anything large and cheap. Do you know of any in the 32-64K area?

Sorry, no. If serial access is acceptable at one side, you could consider a VRAM.

I guess the other option is to use a CPLD chip with internal ram programmed to behave like dual-port ram.

Usually CPLDs contain (much too) few flip flops (=static RAM) to do anything that would deserve the name "RAM". Even 64 bytes RAM to replace a 6530 RRIOT by a CPLD seem to be a problem. FPGAs offer lots of RAM, but they'll probably require some level shifter to 5 volts and some other odds.

Do you have any plans on updating your project? Perhaps if there was enough interest you could take pre-orders for a small run?

The "classic" CHARGEN project found little to none interest. Seems like nobody ever used my schematics to build his own, at least I'm not aware of it. And what's more: I'd like to "finish" my current projects first, like the petSD and the SoftROM. However, if there were a number of people wanting one, I'm afraid I couldn't resist...