24 Jan
2015
24 Jan
'15
5:22 a.m.
-----Original Message-----
From: cctalk [mailto:cctalk-bounces at classiccmp.org] On Behalf Of tony
duell
Sent: 24 January 2015 06:19
To: General Discussion: On-Topic and Off-Topic Posts
Subject: RE: VT101 8085 CPU Fault
is
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
levels
if all devices are momentarily disabled.
This doesn't mean the ROM is good, of course!
it
is
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 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
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.
Regards
Rob