11 Aug
1998
11 Aug
'98
1:15 a.m.
At 10:29 PM 8/10/98 -0500, Doug wrote:
In the last episode of the continuing saga of my little
almost-complete
ET-3400, you might remember that I found the LEDs for it, but not the ROM
chip. Of course, it turns out the ROM chip is pure unobtanium: a special
mask-programmed MCM6830A. Luckily, I found that Heathkit was still in
business, and still supported the ET-3400, so I ordered a ROM chip from
them.
They send me a 2716. Hmm, it's not pin-compatible, so no big surprise
that it doesn't work. So, I called them up. "Oh, you need an adapter
board for that old ET-3400, but we stopped making those things 15 years
ago." It turns out that one of the techs there has the last adaptor board
sitting in his desk drawer. Now the adapter board is mine!
Well, good work finding it!
But it needs a buffer chip. The tech says "I
think it needs a 74LS174".
But that's not a buffer chip, and it's got the wrong number of pins (16
instead of the 14 the socket has).
I'm guessing it really needs something like a '242 or '243. Is there any
way to know for sure without guessing? Am I going to blow anything up by
trying the two chips above? Any other guesses?
Can you trace the circuit of the buffer chip to see at least what pins are
outputs to the 2716 (addresses) and what is from the 2716 to the adapter
(data lines)?
Another choice is a '125. A '242, '243 only have the data inverted from each
other, so if one's the correct chip, the other won't cause logic conflicts,
just wrong polarity signals.
-Dave