VT101 8085 CPU Fault

John Wilson wilson at dbit.com
Fri Jan 23 19:54:03 CST 2015

On Sat, Jan 24, 2015 at 12:29:59AM -0000, Robert Jarratt wrote:
>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.

Based on nothing more than the fact that I know the APL char gen ROM socket
on the VT100's AVO board has jumpers to choose between various chip-select
polarities, and that the default setting (for DEC's mask ROM) differs from
a 2716 or whatever the similar-sized normal EPROM was (since I had to change
them to make one of those work 1,000 years ago), I buy the theory that this
thing may have non-standard chip-select pins/polarities.  Reading as all ones
certainly does sound like it's not being selected properly, just as much as
it could mean a frotzed chip.

>3. Does anyone have a ROM image for the VT101?

Nope but I do have a VT101, so if you get stuck (and someone does know how
to set the CS pin(s)), I'd be willing to try to read it out.  I got the
VT11 char gen PROM out using a LPT port and some perf board so it's OK if
it doesn't correspond to any vanilla EPROM type, as long as the pinout
can be figured out.

John Wilson
D Bit

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