M8650 async board problem

From: Pete Turnbull <pete_at_dunnington.u-net.com>
Date: Sat Dec 22 19:22:12 2001

On Dec 22, 23:15, Tony Duell wrote:

> As the fault wasn't on the M8650, why did they cut that track? Did they
> just never want interrupts?

I've no idea. It wasn't cut by the previous owner; he doesn't know why

> > However the fault, for those who're still reading, wasn't really the
> > itself. There were two problems. At some point, I had unplugged the
> > serial cable from the Berg connector, so there was noise on the serial
> > input, which upset the diagnostics. The second problem was that the
> And adding an 'antenna' (as in the serial cable) actually helps this?

The proper cable has a jumper in the Berg connector to enable the EIA part
of the circuit. It connects the 1489 output to the first gate in the
receiver section. Without the jumper, the input to that gate floats, and
hence picks up all sorts of noise. I wouldn't be surprised if I could pick
Radio 2 up on that :-)

> > +5V was down to about 4.5V, which isn't great for TTL. I'd checked the
> This has caught me so often when repairing DEC machines. The PSUs set up
> fine on no-load and drop to 4V to 4.5V when loaded. And this causes the
> most _amazing_ faults.

Yes, and I should have known better. I realised something of the sort was
amiss when I found that moving cards between front and rear backplanes made
a difference. With everything in the front, the +5V went down to less
than 4.3V.

Pete						Peter Turnbull
						Network Manager
						University of York
Received on Sat Dec 22 2001 - 19:22:12 GMT

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