emmo@moncam.co.uk (Dave Emmerson) (07/27/89)
Aaagh!
Is it just my imagination, or does 10% of the mail in this group really
consist of requests for details of piggy back expansions?
Perhaps if I reply, they might desist?
OK peeps, you asked for it..
First off get hold of 16 DRAMs in Dual In Line packages. These look like
the ones already fitted along the front right of your ST, if in doubt,
quote the section of the device marking which includes the number 256,
and ask for an equivalent part. There are far too many to list them all,
but basic specs are : 256K x 1 page mode DRAM, 150 nanoseconds or faster,
preferably in plastic package (cheaper!), 16 pin DIL outline.
They're not cheap, and prices vary enormously, so shop around.
Each chip has a notch at one end, to show which way up it is fitted.
With the notch uppermost, pin 1 is to the left of the notch, pin 8 is
at bottom left, pin 9 bottom right, and pin 16 top right. Armed with
this information, find pins 4 and 15 on each of your new chips in turn,
and bend them CAREFULLY through 180 degrees without breaking them off.
Bend them as close to the body of the chip as you can. These are the
RAS and CAS pins which work as chip select lines. The chips will normally
be supplied in black anti-static foam. Try to keep them there while you
bend the pins.
Now, one at a time, sit a new chip on top of one of the built-in DRAMs,
with the notch at the same end (!!) and solder each of the other 14 pairs
of pins together. Make sure the chips are accurately lined up, use only
the minimum of fine multicore solder, and watch out for 'bridges' of
solder between adjacent pairs of pins, and solder splashes on the PCB.
This takes care of all the address, data, power, and read/write_ lines.
Lastly, locate the MMU chip. This is a square, 68 pin device, U15 on
early machines, and probably the one nearest the DRAMs on all versions.
This has a pip in the middle of one side to indicate pins 1 & 68.
From the top of the board, they are numbered anticlockwise. Attach one
end of a short (300mm or so) length of thin INSULATED wire to each of
pins 18 (RAS1), 21 (CAS1LOW), and 22 (CAS1HIGH). Prototyping wire is ideal,
particularly the "solder-wrap" type sold in the UK as "Verowire" (R) for
around 5.5 pounds. Don't connect directly to the MMU, take the wires through
a suitable hole, and trace the pins to the underside of the PCB.
The wire from RAS1 should be soldered to pin 4 of ALL the new DRAMs. If you
are skilled enough to do it, a 33 ohm Resistor should be used in series as
near to the MMU as possible to dampen reflections on all 3 of these wires,
but its omission is unlikely to cause problems.
The wire from CAS1LOW should be soldered to pin 15 of the LEFTmost 8 new
DRAMs, and that from CAS1HIGH to the RIGHTmost 8.
That's it. Depending on your confidence and proficiency, it should take
about half to 2 hours. DO try to do it all in one session, and avoid
touching the chips as much as possible, they are static sensitive.
A use ful tip - DISCONNECT the power supply, and attach a lead from the
ground rail around the edge of the board to your metal watch strap. keep the
chips in their foam sitting on a bare area of the ground rail. I'ts not
a static safe workstation, but it's as near as most of you will get.
If you have a hard disk, unhook it before you first try out your handiwork!
If it doesn't work : don't blame me! If your ST runs, but still only with
0.5 MB, check for unsoldered pins, suspect duff chip(s). If it goes crazy,
check for solder shorts, check correct pins used on MMU, take it to your
nearest hardware wiz with a print of these 'destructions'.
Finally, piggybacking 1Mbit DRAMs instead of 256K for 2.5Mbytes :
This is MESSY, the pinouts don't match up well, and several pins need to
be bent through impossible angles. You will need to connect another flying
lead for the extra address from MMU pin 64 to A9 on ALL the 1M drams.
Again the 33 ohm resistor aught to be included as before.
The relevent pinouts follow, I won't go into more detail as I hold the
view that if you can't figure it out from here, you shouldn't attempt to
do it. A useful hint though, apart (perhaps) from A9 (for refresh
considerations), the address pin numbers can be swapped around anyhow
you like, ie A0 can be used as A1 etc..
Viewed from above:
256K DRAM pinout 1M DRAM pinout
notch notch
1 A8 16 Vss 1 Data in 18 Vss
2 Data in 15 CAS_ 2 Write_ 17 Data out
3 Write_ 14 Data out 3 RAS_ 16 CAS_
4 RAS_ 13 A6 4 UNUSED 15 A9
5 A0 12 A3 5 A0 14 A8
6 A2 11 A4 6 A1 13 A7
7 A1 10 A5 7 A2 12 A6
8 Vcc 9 A7 8 A3 11 A5
9 Vcc 10 A4
The above is submitted in good faith, but you use it at your own risk.
If you spot any mistakes, please post to this group first, flame me
later!
Dave E.