VT101 8085 CPU Fault

Robert Jarratt robert.jarratt at ntlworld.com
Sat Jan 24 07:22:05 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

As I can't use my programmer at the moment, I just put the ROM in a
breadboard to try the pins and see what happens. I used the pinout of the
MCM68776 and it seems to correspond exactly. Basically the data outputs
change as I change the address lines when pin 20 is low, which is as per the
datasheet. This is odd, because I told the programmer it was MCM68776, and
yet it read all 1s, whereas when I use it on the breadboard, the ROM does
seem to return 0s as well. I will have to investigate further when I can get
to use the programmer again.



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