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9-Pin D Serial Port RS-232 Cables
To connect to the AT style RS-232 serial port like those found on the PCMCIA Dual card you will need
a cable terminating in a 9-way female D connector. It is sound practice to use cables with screws fitted
that will allow you to fasten the cable securely to the PC card.
In general, you will need to make up a “cross over cable” to correctly interface the PC to the RS-232
port of another computer or device. The cross over cable is simply to ensure that the right signals going
out of one RS-232 port go into the appropriate lines of the other RS-232 port.
9-Pin D Serial Port Connection To Another PC
Suppose we want to connect the AT style 9-pin D serial port to the serial port of another IBM PC. See
Figure 4-3.
1)
Connect the earth lines.
Line 5 of Serial Port 2 to lines 1 & 7 of the other PC.
This gives the two devices a common earth level.
2)
Connect the Transmit and Receive lines together.
Line 3, TXD, Port 2 goes to line 3, RXD, of the other PC.
Line 2, RXD, Port 2 goes to line 2, TXD, of the other PC.
This allows each to receive data transmitted by the other.
3)
Connect the Port 2 DTR line, pin 4 to the other PC DCD, pin 8 and CTS, pin 5, lines.
Also, connect up the other PC DTR line, pin 20 to the Port 2 DCD, pin 1 and CTS pin 8, lines.
This allows the receiving device to signal when it can no longer accept data. The receiving device
sets DTR false when it is unable to receive any more data. The sending device reads DTR on its
CTS and DCD pins. It should stop sending when CTS goes false.
4)
Connect the Port 2 RTS line, pin 7, to the other PC DSR line, pin 6. Also, connect the other PC
RTS line, pin 4, to the Port 2 DSR line, pin 6.
This RTS line is used to let the other device know that it is ready for data exchange.