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mech_mem.doc
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Wrap
Text File
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1980-01-03
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7KB
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122 lines
The memory system was wirewrapped on a 4.5in by 8.5in board perforated on
.1in centers. This is a standard size available from several vendors. A
connector to fit the Amiga expansion connector is mounted to the board such
that the board will stand beside the Amiga extending from the rear up near
the mouse/joystick connectors. The board is slightly higher than the Amiga
but hasn't gotten in the way for me. The memory array (four rows of eight
chips) is at the top of the board. A fifth row contains the buffers for the
array. Next is the connector for the Amiga. Three rows below the connector
are used for timing logic and the clock. The chips all face the Amiga so
that the wire wrap pins of the chips and the connector face out for easy
testing. (I hope this isn't an issue for you, but I did a lot of it.)
The connector is spaced away from the board by placing a 1/2 inch wide
strip of plastic (made from the tubes that wire wrap sockets come in)
between the rows of pins. This leaves just enough room on the pin on the
back side of the board for one wire to be wrapped on the pin. This allows
the connector to reach into the Amiga and leave room for the chips between
the board and the side of the computer.
TOP
+-------+-------+-------+-------+-------+-------+-------+-------+
| H1 | H2 | H3 | H4 | H5 | H6 | H7 | H8 |
+-------+-------+-------+-------+-------+-------+-------+-------+
| G1 | G2 | G3 | G4 | G5 | G6 | G7 | G8 |
+-------+-------+-------+-------+-------+-------+-------+-------+
| F1 | F2 | F3 | F4 | F5 | F6 | F7 | F8 |
+-------+-------+-------+-------+-------+-------+-------+-------+
F | E1 | E2 | E3 | E4 | E5 | E6 | E7 | E8 |
R +-------+-------+-------+--+----+---+---+-------+-------+-------+ B
O | D1 | D3 | D4 | D5 | D7 | A
N +---+-----------+----------+--------+-----------+---------------+ C
T | C O N N E C T O R | | K
+---+---+-------+-------+-------+-------+-------+ C8 |
| C1 | C2 | C3 | C4 | C5 | C6 | |
+-------+-------+-------+-------+-------+-------+---------------+
| B1 | B2 | B3 | B4 | B5 | B6 | B8 |
+-------+-------+-------+-------+-------+-------+-------+-------+
| A1 | A2 | A3 | A4 | A5 | A6 | A7 | A8 |
+-------+-------+-------+-------+-------+-------+-------+-------+
BOTTOM
View of board from wire wrap pin side. You are looking at the right
side of the Amiga if the board is installed. The chip locations are called
out in the schematic and parts list.
The connector is numbered like this:
1.5.........................85 From same view as above.
2...........................86
The schematic calls out pins on this connector like [nn]. If you look in the
hardware manual (preliminary) you may think the connector looks like:
123.........................43
............................86
This is WRONG.
If you cannot find an 86 pin connector with .1in spacing (sometimes called a
43 pin dual row connector) you can cut down a longer connector. I easily
found 100 pin connectors. If you do this be careful when installing the board
that the connector is aligned with the cardedge in the Amiga. It is very
easy to have it slip to the side with one end missing. This will harm the
computer.
When looking at the chips from the back side (as viewed above) the pins are
like this:
1 2 3 4 5 6 7 8
16 15 14 13 12 11 10 9
For 16 pin chips. For other sizes of chip, the numbering goes clockwise as
shown.
All chip sockets, except the 24 pin socket for the MC146818, have a .1uf
capacitor connected between the corner pins 8 and 16 (or 7 and 14 or 10 and
20). There are two techniques that I find convenient. If the type of socket
and the type of capacitor permit, the nicest thing to do is to attach the
capacitor to the socket pins before the socket is pushed into the board. The
socket may permit the capacitor to fit into the cavity usually found between
the rows of pins. The wires will raise the socket only slightly off the
board. If this won't work because of the parts at hand, I usually solder the
capacitors on last, after all wirewrap connections are made. Any other
approach tends to make wirewrapping difficult because the capacitor is in
the way.
I would wrap the power and ground connections first. Make a grid of these
wires such that each ground pin is connected to its nearest neighbors in all
four directions. If your sockets won't accept four levels of wrap, connect
at least three ways so that as much of the grid structure as possible is
maintained. The memory array has many pins that are common to all 32 chips.
These connections are also best made with a grid like pattern. The idea is
to connect all the relevant pins with a minimum of inductance. Since, with
wire wrap, there is no other parameter under your control, redundant
connections are the way to achieve this.
The other connections can be made in any order you like. Minimizing total
wire lengths when connecting several pins is nice, but not strictly
necessary. I made some effort in this direction when I placed the parts and
wired them, but the layout is not optimal in this sense. I don't think it is
necessary to be too concerned with the wirewrapping "rules" for this board.
One thing to attempt to do is to keep wires on the same "level". If you have
four pins (for instance) you connect the first and second, then the third
and fourth, and finally the second and third. Doing things this way means
that when you make a mistake you only have to remove at most three wires to
correct it. You cannot unwrap a wire and re-use it, (if you do, the
connection will probably fail in a few months) so any misconnected wire
must be completely removed. This means any wire covering it must be removed.
Don't daisychain yourself into a situation where many wires must be removed
to correct an error.
There is a variable capacitor in the clock circuit. This is used to adjust
the clock so that it keeps proper time. The capacitor should be mounted so
that it can be adjusted with the board installed and running. Since there
are many styles of capacitor, I cannot suggest any particular method.