21 Nov
2001
21 Nov
'01
10:40 a.m.
You'll want to be careful with the early DRAMs, as they used 3 supply rails, and
your tester of 5-volt-only DRAMs may die from application to a 3-supply DRAM.
Dick
----- Original Message -----
From: "Tothwolf" <tothwolf(a)concentric.net>
To: <classiccmp(a)classiccmp.org>
Sent: Wednesday, November 21, 2001 4:43 PM
Subject: Re: 1771 floppy controller questions
On Wed, 21 Nov 2001, Richard Erlacher wrote:
> From: "Tony Duell" <ard(a)p850ug1.demon.co.uk>
>
> > Mybe RAM. Most disk problems on the M3 and M4 are either the 1793 chip
> > (I've had to replace a couple) or more likely that infernal ribbon cable
> > between the CPU board and the disk controller. If you don't care about
> > your machine being unoriginal, you can use 40 way IDC cable and
> > connectors to replace it (just use 1 row of pins in the connector to
> > replace the 20 pin SIL socket on the CPU board or disk controller).
> > You're only using 20 wires in the cable, which is something of a waste,
> > but it's the easiest fix I've found [1].
>
> For troubleshooting possible RAM problems, it's really easy simply to hook a
> 64Kx8 SRAM on top of the DRAMs, grabbing the addresses before the decoder
and
> multiplexers, and use OR'd nCAS to the DRAMs
as the output enable to the
SRAM.
> You simply disable the data from the DRAMs by
pulling the pins out of the
> sockets. The result is that your system works using the SRAM, yet you can
poke
around all you
like in the DRAM array and the associated timing logic.
I bought a used dip style dram tester set about a year ago. It only cost
me $5, and I figured it might be useful for testing all the old 64k and
256k drams I seem to run into in old pcs. I haven't yet had a chance to
use it yet. Maybe it will help with these old boxes...
-Toth