VT101 8085 CPU Fault

Johnny Billquist bqt at update.uu.se
Fri Jan 23 18:56:08 CST 2015

On 2015-01-24 01:52, Johnny Billquist wrote:
> On 2015-01-24 01:29, Robert Jarratt wrote:
>>> -----Original Message-----
>>> From: cctalk [mailto:cctalk-bounces at classiccmp.org] On Behalf Of Matt
>> Burke
>>> Sent: 20 January 2015 23:20
>>> To: cctalk at classiccmp.org
>>> Subject: Re: VT101 8085 CPU Fault
>>> On 20/01/2015 05:06, Robert Jarratt wrote:
>>>> Do you recall what EPROM you used and how its pinout differed from the
>>>> ROM you were replacing? As far as I can tell there is only one ROM on
>>>> the board, could it be both the character ROM and the firmware?
>>> I used a 2716, but you will probably need a 2764 for the ROM in
>>> question.
>> If you
>>> look at the four ROMs  on the VT100 schematic you will see that the chip
>> select
>>> pins are different for each one. Mask ROMs can be manufactured with any
>>> combination of chip select required whereas EPROMs are fixed. This means
>>> that the four mask ROMs can be installed in any order.
>>> There should be a separate ROM on the board for the character generator.
>>> The part number will be something like 23-018E2-00. It's usually
>>> soldered
>> in.
>>> Matt
>> I probed the pins of the ROM with my scope and some of the pins looked
>> a bit
>> odd, seeming to have 3 voltage levels rather than 2.
>> I took the ROM out of the VT101 and put it in my programmer to try
>> reading
>> it. I had a private suggestion that as a 24-pin 8K ROM chip, it would
>> probably by equivalent to MCM68766, but when I read it with the
>> programmer
>> every single bit came out as a 1. So either the ROM is really bad, or
>> I have
>> chosen the wrong device type. I probed some of the address lines on
>> the 8085
>> and the ROM and they seemed to correspond, so perhaps I did choose the
>> right
>> device type.
>> So:
>> 1. Given the ROM is marked: CN55004N 8232, DEC TP03, 23-028E4-00, and
>> it is
>> 24-pin, is MCM68766 really the right choice?
>> 2. If it was right, is there a modern equivalent? Ebay does not have
>> any of
>> these chips in the UK. It looks like I may have to buy/make a 28->24 pin
>> adapter for a 2764, or ship MCM68766 from the USA.
>> 3. Does anyone have a ROM image for the VT101?
> Given it's "8232", I could suspect that it would be a 4K rom...
> Also, looking at the VT100 and VT103, they have 2K roms, named 8316,
> which really reinforces my suspicion that this would be a 4K rom...

...or I might be confused. I swapped "83" for "82" here. You might want 
to ignore me. :-)


Johnny Billquist                  || "I'm on a bus
                                   ||  on a psychedelic trip
email: bqt at softjar.se             ||  Reading murder books
pdp is alive!                     ||  tryin' to stay hip" - B. Idol

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