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PSI BOX Reference
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RS232 Port Cables.
The RS232 port is the middle port on the back panel of
the PSI BOX. To connect to the PSI BOX RS232 port you will
need a cable terminating in a 25 way female D connector. It is a
sound practice to use cables with screws fitted that will allow
you to fasten the cable securely to the PSI BOX.
In general, you will need to make up a "cross over" cable
to correctly interface the PSI BOX to the RS232 port of another
computer.
Traditionally, making up the cross over cable has been
considered a black art. However, provided you have the pin outs
and handshake requirements of both sides of your RS232
connection, the cross over cable becomes a matter of common
sense.
The cross over cable is simply to ensure that the right
signals going out of one RS232 port go into the appropriate lines
of the other RS232 port.
Connecting To An IBM PC Serial Port.
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Suppose we want to connect the PSI BOX to the serial
port of an IBM PC. See Figures 5-12, 5-13 and 5-14.
1)
Connect up the earth lines.
Lines 1 & 7 of the PSI BOX to lines 1 & 7 of the PC.
This gives the two devices a common earth level.
2)
Connect the Transmit and Receive lines together.
Line 2, TxD, of the PSI goes to line 3, RxD, of the PC.
Line 3, RxD, of the PSI goes to line 2, TxD, of the PC.
So each receives the data transmitted by the other.
3)
Connect up the PSI DTR line, pin 20 to the PC DCD, pin
8 and CTS, pin 5, lines.
Also, connect up the PC DTR line, pin 20 to the PSI
DCD, pin 8 and CTS, pin 5, lines.
This allows the receiving device to signal when it can no
longer accept data. The receiving device sets DTR false
Page 44
RS232 Port
Chapter 5
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