VT101 8085 CPU Fault

tony duell ard at p850ug1.demon.co.uk
Sat Jan 24 00:18:35 CST 2015

> 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.

Which pins? 

The address pins are inputs to the ROM, so if they 'look odd', the problem is 
likely to be something else, such as whatever chip drives the address bus.

The data pins go to a shared bus (several 3-state devices can drive it, only
one is enabled at a time), and may well appear to have intermediate voltage
levels if all devices are momentarily disabled.

This doesn't mean the ROM is good, of course!

> 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

If all the bits are 1's then either the ROM is totally dead or more likely it is never
being enabled. The chip select input(s) on mask ROMs can often be selected
(by the final metalisation layer that determines the ROM contents) to be either
active-high or active-low. EPROM ones are active low. So it is possible your
programmer is disabling the chip.

If the ROM was all 1's then I would expect the 8085 to be exectuing FF's = 
RST 38h. Since location 0038 is very likely to be in the ROM of a simple
8085 system like this, it will just keep on exectuting RST 38h, pushing
39 00 onto the stack each time (and thus filling writable memory with
those bytes). This is not what your VT101 is doing AFAIK, so I would
think the ROM is not all 1's


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