Re: Restoring a NorthStar Horizon, problems with SRAM board

On May 21, 11:41 am, Herb Johnson <herbrjohn...@xxxxxxxxx> wrote:
On May 20, 2:01 pm, lync...@xxxxxxxxx wrote:

OK, I hear you on the oscilloscope. I have bought one on Ebay and it
will be a few days for it to arrive. It is a dual trace 40MHz model
which should be sufficient for this task.

Andrew Lynch

Using an oscilloscope is not so bad. In brief, you can use it as a
logic probe (trigger on the incoming signal) and just check those
various chip pins for activity. Unlike a volt meter or LED based logic
probe, you can look at changing logic levels (or lack of change). A
"level" of about 2.5V is an open TTL input: if it's supposed to be
connected - and all TTL inputs SHOULD be connected to something - then
you know you have a bad socket or broken trace. Those "barely driving"
outputs may also suggest you have some bad INPUT somewhere on the
line; but it's possible the drivers are at fault instead. If they are
bus drivers, the bus may have had some "fault" on it, a -5V across
those drivers for instance, to damage them.

You can also tie the second channel of the 'scope to some reference -
the Z80 clock, the Z80 M1, whatever - and trigger off that refernece.
That's how you track down timing problems, in circuits with marginal
designs. The Northstar I believe does not have marginal designs. And
you can trigger off the approprate "chip select" of a given IC and
watch the activities of the outputs. That's useful for tracking down
program actions versus desired activities, such as that floppy
controller board. Since the N* controller is all discrete chips, it's
harder to track than a FDC chip.

Otherwise your posts look through. Keep up the good work, you'll have
this Northstar up in due course!

Herb JOhnson

PS thanks for the reference link on 2708's. I'll add that to my Web
site, with the linked page owner's permission!

Herb

Herbert R. Johnson, New Jersey USAhttp://www.retrotechnology.com/herbs_stuff/ web sitehttp://www.retrotechnology.net/herbs_stuff/ domain mirror
my email address: hjohnson AAT retrotechnology DOTT com
if no reply, try in a few days: herbjohnson ATT comcast DOTT net
"Herb's Stuff": old Mac, SGI, 8-inch floppy drives
S-100 IMSAI Altair computers, docs, by "Dr. S-100"

Herb, Dave, and others,

I posted a response on this thread yesterday but Google Groups refused
to post it. Unfortunately, I had to go on a business trip so it will
be a little while for an update. I kept a copy and will post when I
get back.

A couple of quick questions though. Dave, would you email me the
assembler source for that ROM only monitor you mentioned earlier?

After reading the MDS-A floppy drive controller manual, I noticed a
short note. What are the symptoms if the SA-400 floppy disk head
carriage gets out of the spiral groove track? Does it make the floppy
drive behave in a halting or choppy manner? How do you know if you
have the problem and how is it fixed?

Thanks!

Andrew Lynch

.

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