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Old 06-07-2007, 02:01 AM
Grant Stockly Grant Stockly is offline
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Join Date: Jan 2005
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From: Grant Stockly
Date: Sat, 02 Jun 2007 20:41:05 -0800
Subject: [Altair Computer Club] 88-PPG fixed with story


Using the data from the logic analyzer I traced out the problem with the
88-PPG and fixed it. This makes me happy and I'm sure Robert too. :
) Btw Robert if you are reading this please call or e-mail me! : )

I've uploaded all data sheets for the ICs listed and a ton more stuf in the
file area. All you digital pack rats should go take a look at it. : )

There is also a picture of a 88-PMC ROM card with a ZIF socket
installed. This allows me to swap ICs without removing the card.

===

A0 is stuck high. This causes no odd bytes to be programmed and all even
bytes to be programmed twice with data from the associated odd byte. : (

There is a section on the motherboard where A0 is buffered twice. The
input to the circuit is 0-5v TTL and the output is -48-0v. A0 is stuck
high before this. A0 is generated by a 7400 positive NAND gate, 4Y (pin
11). Pin 13 is high. Pin 12 needs to be high in order to make pin 11
low. Pin 12 goes to a 8212, an 8 bit I/O port, pin 4 where it is stuck low.

Pin 4 on the 8212 is DO1 (data out bit 1 of 8). Pin 3 is the associated
DI1 (data in bit 1 of 8).

There is no activity on pin 3 of the 8212. The 8212 is not bad.

Pin 3, DI1, is shared on all 3 of the 8212s. If it is stuck low then D0
should be stuck low too. I checked the wave form and D0 NEVER goes
high. Hmmm... : ) A0 stuck high, D0 stuck low. This makes sense since
the data lines are inverted compared to the data.

Pin 3 on the 8212 goes to pin 2 of a 74LS14 a Hex Schmitt-Trigger
Inverter. Pin 2 is an output, pin 1 is its input. This is one of the
tarnished ICs... Nothing at all on pin 1.

Pin 1 on the 74LS14 connects to R11, R12, and pin 9 on the system interface
cable. No signal there either. There is no signal on pin 9 of the S-100
interface card either.

The problem is in the interface card. Its pretty simple. : )

Pin 9 on the interface cable goes to:
-pin 24 on an unidentified empty socket.
-pin 3 of IC D, a 74367 a Hex Buffer Driver with tri state output
-pin 6 of IC G, a 8836 (quad NOR gate, sorry no data sheet)

The 8836 pin is an input, so unless it is shorting then it isn't the
problem (I have seen this...).

The legs on IC D are tarnished a nice dark black color. : ) Pin 3 is an
output, pin 2 is the input to that buffer. Pin 3 on the IC socket was not
soldered very well at all. Looked like someone blew their nose on
it. This is weird because the rest of the board is nice and shiny. When
it heated up it must have pulled tight against the board. I noticed that
the when sticking a wire into the socket it had no resistance and when
pulling it out it varied between 200 and 5 ohms. : )

It works now! : ) I just copied the PPD ROM and used it to make copies of
other ROMs. : )

Grant
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