VT101 8085 CPU Fault

Diane Bruce db at db.net
Sun Jan 18 18:31:07 CST 2015

On Mon, Jan 19, 2015 at 12:00:22AM -0000, Robert Jarratt wrote:
> > >
> > > A remote possibility is the boot ROM is b0rked and it's simply running
> > > around like a chicken without a head.
> > > That is something a good logic analyser could tell you.
> > 
> > If the RD pin never gets active, the contents of the ROM will not even be
> > presented to the CPU, so while it's possible the ROM is borked, that is not the
> > problem we're trying to fix right now.
> > 
> I just found something interesting, the pin activity on the address lines, RD, WR, ALE etc appears normal for a bit, and only after a few seconds does it all become inactive.
> Perhaps this really is an indication that the ROM is bad, if it is getting into some kind of loop where it does no external activity. I don’t really know the 8085 to know whether that is possible.
> Regards

If the VT-100 used an EPROM and not a masked ROM I'd suspect bit rot.

A quick glance at this

makes no mention of EPROM vs. masked ROM, but it looking at this manual
it is possible you have bad ram as well. In that case, the LEDs should
signal this.

> Rob

- db at FreeBSD.org db at db.net http://www.db.net/~db

More information about the cctalk mailing list