VT101 8085 CPU Fault

Robert Jarratt robert.jarratt at ntlworld.com
Mon Jan 19 15:26:57 CST 2015

In my latest tests I found that the CPU does indeed seem to HALT, according to S1 and S0 (both go low). Sometimes this is immediate, other times it happens after 15-20 seconds. When it goes for longer it displays more random characters on the screen and lights more lights on the keyboard. But it never lights all the lights and it does not sound the bell. This happened even after reseating the only socketed chip, which is marked CN55004N 8232, DEC TP03, 23-028E4-00, I have no idea what it is as I can't find any info on it.

The random nature of the halt is a concern. If it was bad ROM I would expect it to always fail in the same place, unless the ROM is somehow marginal and gives different results every time.



> -----Original Message-----
> From: cctalk [mailto:cctalk-bounces at classiccmp.org] On Behalf Of Robert
> Jarratt
> Sent: 19 January 2015 20:48
> To: 'General Discussion: On-Topic and Off-Topic Posts'
> Subject: RE: VT101 8085 CPU Fault
> > -----Original Message-----
> > From: cctalk [mailto:cctalk-bounces at classiccmp.org] On Behalf Of sandy
> > hamlet
> > Sent: 19 January 2015 19:38
> > To: General Discussion: On-Topic and Off-Topic Posts
> > Subject: Re: VT101 8085 CPU Fault
> >
> > Using a scope, look at the address lines and/or data lines at power
> > up, just after system reset.
> > The pulses will probably vary in width but should all be the same amplitude.
> > If you see something like stair stepping: where a pulse starts out at
> > 5 volts and drops, for example to 3 volts and then drops to 0 volts,
> > then you might have two devices trying to access the same line at the same
> time.
> > Devices on the same line may include a ram, rom, I/O ports, etc.
> > This could cause the processor to "hang up" and act erratic.
> > A schematic of the board would be helpfull if you have one.
> >
> Sadly, that is the one thing that does not seem to be available. If anyone does
> know of a print set for the VT101 please let me know.
> Thanks
> Rob

More information about the cctalk mailing list