VT101 8085 CPU Fault

Robert Jarratt robert.jarratt at ntlworld.com
Sat Jan 24 05:07:41 CST 2015

> -----Original Message-----
> From: cctalk [mailto:cctalk-bounces at classiccmp.org] On Behalf Of tony
> Sent: 24 January 2015 06:19
> To: General Discussion: On-Topic and Off-Topic Posts
> Subject: RE: VT101 8085 CPU Fault
> > 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
> 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
> 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
> your programmer is disabling the chip.
> If the ROM was all 1's then I would expect the 8085 to be exectuing FF's =
> 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
> bytes). This is not what your VT101 is doing AFAIK, so I would think the
ROM is
> not all 1's

You're right, I did think that it seemed odd that it would appear to be all
1's when the CPU does appear to do things, and that the behaviour is random,
suggesting that the ROM can't possibly be all 1s.

I was planning to put the ROM on a breadboard so that I can play with its
connections to the programmer, but, overnight, the old PC I use to connect
the parallel port programmer has developed what looks like a memory fault. I
don't have any spare memory so it will be next weekend now before I can get
new memory and try again. :-(



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