VT101 8085 CPU Fault

Robert Jarratt robert.jarratt at ntlworld.com
Fri Jan 23 18:29:59 CST 2015

> -----Original Message-----
> From: cctalk [mailto:cctalk-bounces at classiccmp.org] On Behalf Of Matt
> 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
> 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
> 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.


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?



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