PCJR mobo so close to being working!

Hardware questions and modifications

PCJR mobo so close to being working!

Postby VERAULT » Thu Nov 19, 2020 2:35 pm

214332.jpg
Dead 1503723
(101.16 KiB) Not downloaded yet

So I decided to take a crack at a PCJR board that was not working I had on my shelf for a while. What it is doing is not displaying text graphics of any kind. Technically the machine is booting and beeping as intended but there is no text or graphics but blocks where there should be graphics and nothing where there should be text.

Alas today using a good board, I determined the fault is the XE 1503723 IC just behind the connector for the Infrared PCB on the board. When I swapped chips from the good board the machine had perfect graphics, and when I tried the faulty IC in the good board is displayed the same bad onscreen graphics.

It has been explained to me that this is the character ROM made by Motorola, and IBM used it in other machines like the 5140 portable. But I haven't been able to find a source to buy one.
It was also mentioned that it might be compatible with 2716 or 2732 EPROMs and I would like confirmation on that. IS this true and if so does someone have the BIN file So I can burn a new one?
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Re: PCJR mobo so close to being working!

Postby VERAULT » Fri Nov 20, 2020 10:57 am

Am I wrong about this being a character ROM? Any help would be appreciated.
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Re: PCJR mobo so close to being working!

Postby VERAULT » Sun Nov 22, 2020 8:06 am

Anyone? Any information at all would be appreciated!
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Re: PCJR mobo so close to being working!

Postby Brutman » Sun Nov 22, 2020 1:10 pm

It's a plain old ROM. The technical reference lists it as MCM28A316E or equivalent device. It has a 350ns access time and is compatible with 2716 and 2732 EPROMs. If you have 256 characters and each is an 8x8 bit pattern then you just need 8 bytes per character, or 2KB of storage total.

PC Enterprises sold a "thin font" modification that was a replacement EPROM with two different character sets; the standard character set and a modified one that was supposed to be easier to read. They used 4KB and tied one of the address lines to a switch that let you choose which part of the EPROM you wanted active.
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Re: PCJR mobo so close to being working!

Postby VERAULT » Sun Nov 22, 2020 4:20 pm

Thank you Mike but does anyone have a copy of the ROM in Bin file format?
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Re: PCJR mobo so close to being working!

Postby DoctorOctal » Sun Nov 22, 2020 5:03 pm

VERAULT wrote:It has been explained to me that this is the character ROM made by Motorola… But I haven’t been able to find a source to buy one.


Yes, this is the character ROM. It’s a mask ROM, meaning it’s programmed at the factory according to the customer’s (IBM’s) specifications. Thus Motorola part number MCM68A316E became IBM part number 1503723. These days the only way you’d be able to buy one of these is as part of a PCjr or as something pulled from a PCjr.

More likely you’d want to make a new copy using an EPROM.

It was also mentioned that it might be compatible with 2716 or 2732 EPROMs and I would like confirmation on that.


Yes, those EPROMs are drop-in compatible. You can read that in the PCjr technical reference (page 2-49) and the Motorola data book (page 6-34).

They are old chips that haven’t been manufactured in years. You can still buy old stock from Jameco (2716, 2732), but I’ve read online that they tend to be unreliable, even from such reputable sources. Perhaps they’d still be worth a try. Because these are UV-erasable chips, you’d need a UV eraser in addition to an EPROM burner if you wanted to leave room for trial and error.

Alternatively you could use an EEPROM that’s still in production, such as an AT28C64B. That would be more reliable and more conveniently erased. On the other hand, it’s a 28-pin device and would require that you make an adapter to fit the 24-pin socket.

does someone have the BIN file So I can burn a new one?


If you have the means to burn an EPROM, you have the means to read the image of the good ROM you already have.

While you’re at it, however, you might want to get an image of the CGA/MDA character ROM and make yourself a thin font mod. If you’re burning a new character ROM anyway, you’re already 90% of the way there. PC Enterprises just copied both the thin and thick fonts from the CGA ROM (the PCjr’s ROM only had the thick version), burned them on a 2732 EPROM, and attached a wire with a grabber clip to address line 11 (pin 21) for the sake of selecting which half of the ROM to use. You can see the details in the documentation. If you set yours up the same way, you could use PC Enterprises’ utility for switching the font on the fly.
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Re: PCJR mobo so close to being working!

Postby Brutman » Sun Nov 22, 2020 5:17 pm

Going slightly off topic here ... I'm going to confess to not having an EPROM programmer of any sort. Is there a FAQ somewhere about how to even get started?

I know the theory. I've just never gotten around to getting one. For a while the Willem was the unit to get, but that's ancient history now.
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Re: PCJR mobo so close to being working!

Postby DarkStar2032 » Sun Nov 22, 2020 6:52 pm

The XGecu TL866II was recommended to me. The cost was about $55.00US at the time.

https://www.ebay.com/i/223232860577?chn ... 34fe7e717f

I plan to eventually use it for PCjr cartridge roms using a zif-socket.
Hi-ho, KERMIT the computer file transfer/management protocol here.
And welcome to NULL MODEM Street.
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Re: PCJR mobo so close to being working!

Postby DoctorOctal » Mon Nov 23, 2020 2:09 am

Brutman wrote:For a while the Willem was the unit to get, but that’s ancient history now.


XGecu’s TL866 “MiniPro” series seems to be a favorite among hobbyists in recent years. It’s Chinesey but fairly straightforward to use. Dave Jones of EEVblog reviewed the TL866CS in 2013 and found it good for the price.

Adrian Black uses one in a number of his videos. See this episode, for example, starting around 14:42.

I have a TL866II Plus and, so far, have only used it for dumping ROMs. The software comes with a database of pinouts and parameters for thousands of different chips, but you’ll still run into ones it doesn’t explicitly support. When I read my PCjr BIOS ROMs, for instance, it didn’t have the MK38000 or TMM23256P in the database. I was able to read them as AM27256 chips after disabling the pin detection, however.

The subject of programming 2716s and 2732s brings up another issue: the older XGecu programmers don’t provide the 25 V programming voltage needed for some vintage NMOS chips. The TL866A and TL866CS can do 21 V, but the TL866II Plus regressed to only 18 V. Some people have skated by with lower programming voltages, made adapters for higher-voltage programming, or used CMOS variants (such as a 27C32) with lower voltage requirements. The latest programmer, the XGecu T56, goes up to 25 V for programming but apparently loses support for a few other things.

Is there a FAQ somewhere about how to even get started?


I’m not aware of a FAQ, as such, but the videos linked above will give you the flavor. And here are some other links I’ve collected:

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Re: PCJR mobo so close to being working!

Postby VERAULT » Tue Nov 24, 2020 2:45 pm

Here is a dump from a working PCJR character ROM:
Anyone wants to try writing it and verifying please let me know. I don't have any 2716 or 2732, but I have some on order.
Attachments
pcjrcharrom1.zip
PCJR CHAR ROM
(1.21 KiB) Downloaded 22 times
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