23 Jan
2015
23 Jan
'15
4:29 p.m.
-----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:
If you
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. 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?
Thanks
Rob