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Recreating 8250 ROMs

snuci

snuci

Veteran Member
Joined
Nov 22, 2012
Messages
1,558
Location
Richmond Hill, Ontario, Canada
Hello,

I have two CBM 8250 dual floppy drives and both had bad ROMs. Luckily, one had a bad 901887-01 (UL1) and the other had a bad 901888-01 (UH1). I was able to get one drive working properly. I recreated both of the ROMs with two MCM69766C EPROMs but they don't work. When turned on, the drive had all lights remaining on and no activity. To confirm it's the EPROMs, I switched them to the working drive and it exhibited the same behaviour. Switching both ROMs to the bad drive made that drive work. I also verified the ROM images from two sources.

Are these the wrong EPROMs?
 
snuci said:
Hello,

I have two CBM 8250 dual floppy drives and both had bad ROMs. Luckily, one had a bad 901887-01 (UL1) and the other had a bad 901888-01 (UH1). I was able to get one drive working properly. I recreated both of the ROMs with two MCM69766C EPROMs but they don't work. When turned on, the drive had all lights remaining on and no activity. To confirm it's the EPROMs, I switched them to the working drive and it exhibited the same behaviour. Switching both ROMs to the bad drive made that drive work. I also verified the ROM images from two sources.

Are these the wrong EPROMs?
Click to expand...

If I remember correctly, the 2364 ROMs used in 8050/8250 were programmed with active high CS, so if you used an MCM68766 (not 69766 that I couldn't find any datasheet if it exists), it's activated on the wrong level, since it comes with active low CS.
You can either make an adapter to incorporate a small 74AHCT1G04 gate to invert the CS going to an EPROM/EEPROM (like I did in similar occasions) or add a piggybacked inverter on the EPROMs.
BTW: how did you read the original ROMs if they indeed were using an active high CS?

Frank
 
Thanks guys. Sorry for the typo. It is, in fact, an MCM68766. Can I use a normal 7404? If so, I'll give it a shot.

I downloaded the ROM images from two different websites. I didn't read the ones I had because I wasn't 100% sure the MCM68766 was compatible so I didn't want to ruin the working ones I had.
 
snuci,
The MCM68766 should work fine, if you can swap the Chip Select (E* Pin has to be LOW for ENABLE).
Likewise it has to be Vih for the Chip to be disabled. I've attached that from the Datasheet. Depending
on your circuit you may have to add a PULLUP resistor from +5VDC to that CD Pin to make sure it
goes HIGH. I had that exact problem on my TES-80 Model 1 when I replaced the Version 1.1 ROM's
with the Texas Instrument TMS-27xx EPROMS. You should be able to verify that the CS is going
HIGH when the Chip is not selected. If just may not go high enough like my Model 1.

Larry


Vih.jpg
 
snuci said:
Thanks guys. Sorry for the typo. It is, in fact, an MCM68766. Can I use a normal 7404? If so, I'll give it a shot.

I downloaded the ROM images from two different websites. I didn't read the ones I had because I wasn't 100% sure the MCM68766 was compatible so I didn't want to ruin the working ones I had.
Click to expand...

Ok it makes sense, you never read the original ROMs ;)
You can use a 7404, but I would use LS04 or HCT04. It won't look pretty though. I also made universal 23xx replacements that can be configured for XX=16/32/64 and any combination of chip select polarities, but I've run out of PCBs and anyway the demand is non existent so far, so I did not order more.
Frank
 
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