The Signetics 2650 - The MESSUI Place

How to expand your 2650 Minicomputer
ABOVE: The position of the 10 sockets
can be scaled from this photograph.
Mount them from the inside of the
chassis.
DATA
IN
Issionto
The wire leading from pin 11 of the
74LS38 (the WRP-bar signal), must be
extended to the G2-bar input of the
74LS138 on the expansion board.
The two power supply rails are
available near the three terminal
regulator. Make sure that you do not
get the polarity wrong.
Once you have finished and checked
all the interconnections between the
two boards, you can wire up the I/O
sockets. We used 15-way Cannon D
Subminiature plugs and sockets. The
connection scheme we used is shown
in the accompanying diagram.
The eight pins in a row are used for
the data connection, with the remaining
seven pins used for t ontrol and
power supply signals. The +5V and OV
signals are the main supply rails, while
the +10V and ---10V signals can be
derived from the power transformer
using a bridge rectifier and two electrolytic
capacitors.
The strobe signal is obtained from
the expansion board, while the control
signals can be selected from the other
76 ELECTRONICS Australia, November, 1978
BELOW: The overlay diagram is used
both to place the components on the
PCB, and to make the connections
between the PCBs.
+5V
cc
z
0
O
cc
z
0
D INPUT • C INPUT D OUTPUT C OUTPUT
ports as desired. For example, to run
the OP-80A paper tape reader, a ninth
data input is required, so this can
become one of the control signals.
Once all the wiring is completed, and
has been checked thoroughly, testing
can commence. This can be done
before the board assembly is replaced
in the case. Fit a heatsink to the
regulator tab, and reconnect the three
wires from the transformer
Monitor the +5V rail, and switch on.
If it does not rise immediately to the
correct value, switch off and trace and
rectify the fault. Assuming all this is
well, connect up your terminal, and
check, that Pipbug and whatever RAM
is fitted is working correctly.
The output ports can be tested by using
a small routine of instructions to
write data to them and checking that it
appears on the appropriate output pin
with a multimeter. You can use the Pipbug
BIN routine to get data bytes from
your terminal, the BOUT routine to
echo them, and the WRTC or WRTD instructions
to transfer them to the out-
La
0
cc
0
Q
z
0
0
z
0
OV
1 •
6 I I I I 1
7 1 5 4 3 2 1 0
DATA LINE
FEMALE 15-WAY CANNON CONNECTOR
ON CPU CHASSIS VIEWED ON WIRING SIDE •
ABOVE: This is the suggested wiring
diagram for the 15way Cannon connectors
we used for the 10 sockets.
BUFFERED
DATA
OUT
Cout
STROBE
-I-
5V
PAGE 0
.------,40441.-PAGE 2 -
---:
PAGE 1
PAGE 3
put ports.
Similarly the input ports can be
tested by reading them via REDC and
REDD instructions, and using the BOUT
routine to print the byte obtained on
the terminal. With nothing connected
to the inputs, you should get hexadecimal
FF from both ports.
Now use a clip lead to ground one input
pin at a time, and check that the appropriate
byte is displayed. For instance,
shorting the bit 0 data input
should change the display from FF to
FE.
Once you are satisfied that all facets
of the unit are operating correctly, the
board assembly can be fitted to the
case. Use of a magnetised long blade
screwdriver will be found helpful in this
operation, if you are using steel screws.
In the next article, we will give details
of the 8K RAM board and the additional
power supply components required
with it. This additional supply
will also provide the +10V and —10y
unregulated voltages mentioned
previously.