OOPS! I misread the previous post . . . it does say 1k x 4 bits.
The only 22-pin 4K-bit sram I remember from those days is one from EMM-SEMI,
but the number escapes me. Nevertheless, the only SRAM I remember in a
22-pin package other than the EMM-SEMI parts, was the 256x4 2101. There
were plenty of non-multiplexed DRAMs in that package, but few SRAMs.
If you have 22-pin parts and the memory diagnostic is failing on a 4K-byte
boundary, it's likely the parts are something wierd. The conclusion that
Philip has reached here is quite plausible. I'd explore his proposed
solution before going any further.
MOS Technology did build a few oddball parts, but I don't remember seeing
any of their parts as being THAT odd. My MOS data books don't include any
memory parts, though they did build some ROMs at that time.
Dick
-----Original Message-----
From: Philip.Belben_at_powertech.co.uk <Philip.Belben_at_powertech.co.uk>
To: classiccmp_at_classiccmp.org <classiccmp_at_classiccmp.org>
Date: Tuesday, March 21, 2000 10:51 AM
Subject: Are you sure the 6550 is dead? (was: Mos Technology RAM wanted)
>
>> I think the basic number you're looking for is "2101" from the same 1K
>> series as the famous "2102" which is a 16-pin 1kx1 with separate in and
out.
>> The 2101's I have are not fast enough to meet the 200ns spec. However,
not
>> much of anything that was readily available at the time the PET model
2001
>> came out was that fast. Either it was quite a bit faster, e.g. 2147,
2115,
>> etc, or it was slower, e.g. 2114, 21L02, TMS4044 etc, which were
typically
>> 450 ns at that point in time. Those 450 ns parts worked handsomely with
the
>> 1 MHz 6502. Perhaps you'd be able to use a 2101.
>
>I would have thought a 2101 was far too small. These are 4kbit chips.
>
>I agree 200ns probably isn't essential - my PET of that date uses 450ns
2114s.
>On the 1MHz 6502 you have 500ns between the two clock edges that govern RAM
>timing - one guaranteeing a valid address, the second latching in the data.
>
>(FWIW there were FOUR motherboard designs for the early PETs, based on all
>permutations of 2114 or 6550 RAM and 2316 or 6540 ROM. Mine is late for an
old
>style PET, and has 2114 and 6540)
>
>>>> > One of the MCS6550 RAMs has gone west. Does anyone have a spare, or
an
>>>> > equivalent, for sale? It's a 22-pin 1024 x 4 200ns static RAM.
>>>>
>>>> I'll have to check the RAMs in the spare PET that I
>>>> keep in the garage. Can't remember whether they're SRAMs
>>>> or DRAMs in the bigger PETs.
>>>
>>>Only the oldest 2001-x PETs use those SRAMs. I guess it's time to build
>>>that upgrade board (a PAL, a pair of EPROMS, and some 6264s, a 62256, or
>>>some cast-off PC cache).
>
>That's right. When they revised the ROM code, they revised the motherboard
so
>that all ROMs were 2332s (and the sockets would take 2732s) and all RAM was
>dynamic.
>
>The thing that bothers me is that it says 3071 bytes free. This is EXACTLY
the
>number of bytes free you get on a 4K PET.
>
>My advice - Identify the suspect pair of chips (remember these are 4-bit
wide
>parts) - I have the circuit diagrams if you want - and swap them with the
>corresponding pair for the top 1k of RAM. You should then get the message
"6143
>BYTES FREE" when you power up. (Or try swapping them with the video RAM -
>you'll soon see if the chip is dead!)
>
>I suspect, however, you will still get 3071 BYTES FREE.
>
>Let me explain. On these early PETS the top four address lines come from
the
>processor and go straight into a 74154 decoder, AND NOWHERE ELSE. This
outputs
>sixteen block select lines - one for each 4k of memory. Your PET is
failing
>memory test at exactly the point where it passes into the second 4k block
from
>the first.
>
>In short, I think a failure at the block boundary is too much of a
coincidence.
>I'd trace out what this line is doing from the 74154 (there is only one,
and
>it's 24 pin, so you can't miss it!) to the RAM. Failing this, I'd suspect
>something else in the chip select logic. (It could, of course be a failure
that
>affects a whole chip, but I'd try the other things after you've swapped RAM
>chips if the BYTES FREE doesn't change)
>
>As I said, I can lend you my copy of the circuit diagrams if you haven't
already
>got one.
>
>Philip.
>
>
>
>
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Received on Tue Mar 21 2000 - 12:16:57 GMT