VT101 8085 CPU Fault

Johnny Billquist bqt at update.uu.se
Fri Jan 23 18:52:22 CST 2015

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...


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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