Serial Connectors and Pin Assignments
Connecting peripheral devices such as digitisers and plotters to computers can be time consuming. This is largely due to manufacturers different interpretations and implementations of the serial RS232C standard. Part of the problem occurs because RS232C was only intended to describe communications between computers and modems, not other devices such as printers.
This set of cable diagrams is a collection of configurations that have been used successfully at a number of sites in the past. Unfortunately due to the confusion over the RS232C standard these configurations are only suggested as possible solutions.
The RS232C standard does not actually define the connectors between computer and cable. This only adds to the confusion as there are a number in use. The most common are the 25 pin D connector and the 9 pin D connector. Others in use are the round DIN connector and the clip style RJ connectors (common on telecommunication equipment).
25 Pin D connector Pin Assignments
| 1 | PG | Protective Ground |
| 2 | TD | Transmit Data |
| 3 | RD | Receive Data |
| 4 | RTS | Ready To Send |
| 5 | CTS | Clear To Send |
| 6 | DSR | Data Set Ready |
| 7 | SG | Signal Ground (Common Return) |
| 8 | DCD | Data Carrier Detect |
| 20 | DTR | Data Terminal Ready |
| 22 | RI | Ring Indicator |
Note:The other pins not described are rarely used and then usually only as secondary lines.
9 Pin D connector Pin Assignments
| 1 | DCD | Data Carrier Detect |
| 2 | RD | Receive Data |
| 3 | TD | Transmit Data |
| 4 | DTR | Data Terminal Ready |
| 5 | SG | Signal Ground (Common Return) |
| 6 | DSR | Data Set Ready |
| 7 | RTS | Ready To Send |
| 8 | CTS | Clear To Send |
| 9 | RI | Ring Indicator |
Note:The Protective Ground pin is not included but sometimes the cable shield is attached to the actual D pin connector.
Modifications to Presented Diagrams
The following diagrams are educated suggestions that have worked in past situations. You may need to modify them slightly to fix your communication problem. The most common modification would be to reverse pins 2 and 3 from their described direction.
Cable Diagram 1
Uses: Universal 9 to 25 Pin Converter
Computer End Device End No Pins: 9 No Pins: 25 Sex: Female Sex: Male 1 PG DCD 1 ----------------------- 8 DCD RD 2 ----------------------- 3 RD TD 3 ----------------------- 2 TD DTR 4 ----------------------- 20 DTR SG 5 ----------------------- 7 SG DSR 6 ----------------------- 6 DSR RTS 7 ----------------------- 4 RTS CTS 8 ----------------------- 5 CTS RI 9 ----------------------- 22 RI |
Note:Sometimes the cable shield will be used to connect the PG (Protective Ground) pin of the 25 pin connector to the chassey of the 9 pin connector.
Cable Diagram 2.a
Uses: GTCO Digitiser
Houston Instruments Digitiser
Calcomp Plotters
Some HP Plotters
Computer End Device End No Pins: 25 No Pins: 25 Sex: Female Sex: Male PG 1 1 PG TD 2 ----------------------- 2 TD RD 3 ----------------------- 3 RD RTS 4 -| |- 4 RTS CTS 5 _| |_ 5 CTS DSR 6 -| |- 6 DSR DCD 8 -| |- 8 DCD DTR 20 _| |_ 20 DTR SG 7 ----------------------- 7 SG RI 22 22 RI |
Note:This is a pretty universal cable where hardware handshaking is to be used. It will probably solve most cabling problems. See Diagram 2.b for a 9 to 25 pin version or Diagram 2.c for a 9 to 9 pin version of this cable.
Cable Diagram 2.b
Uses: 9 to 25 pin version of Diagram 2.a
Computer End Device End No Pins: 9 No Pins: 25 Sex: Female Sex: Male 1 PG DCD 1 -| |- 4 RTS DTR 4 -| |- 5 CTS DSR 6 -| RD 2 ----------------------- 3 RD TD 3 ----------------------- 2 TD SG 5 ----------------------- 7 SG RTS 7 -| |- 6 DSR CTS 8 -| |- 8 DCD RI 9 -| |- 20 DTR 22 RI |
Cable Diagram 2.c
Uses: 9 to 9 pin version of Diagram 2.a
Computer End Device End No Pins: 9 No Pins: 9 Sex: Female Sex: Male DCD 1 -| |- 1 DCD DTR 4 -| |- 4 DTR DSR 6 -| |- 6 DSR RD 2 ----------------------- 2 RD TD 3 ----------------------- 3 TD SG 5 ----------------------- 5 SG RTS 7 -| |- 7 RTS CTS 8 -| |- 8 CTS RI 9 9 RI |
Cable Diagram 3.a
Uses: HP Plotters
Computer End Device End No Pins: 25 No Pins: 25 Sex: Female Sex: Male PG 1 1 PG TD 2 ----------------------- 3 RD RD 3 ----------------------- 2 TD RTS 4 ----------------------- 8 DCD DCD 8 ----------------------- 4 RTS CTS 5 ----------------------- 20 DTR DSR 6 -| DTR 20 ----------------------- 5 CTS |- 6 DSR SG 7 ----------------------- 7 SG RI 22 22 RI |
Note:This is a more conventional interpretation of Diagram 2.a which contains loop backs. This cable too should solve many other hardware handshaking problems. See Diagram 3.b for a 9 to 25 pin version of this cable.
Cable Diagram 3.b
Uses: 9 to 25 pin version of Diagram 3.a
Computer End Device End No Pins: 9 No Pins: 25 Sex: Female Sex: Male 1 PG DCD 1 ----------------------- 4 RTS RD 2 ----------------------- 2 TD TD 3 ----------------------- 3 RD |- 5 CTS DTR 6 ----------------------- 6 DSR SG 5 ----------------------- 7 SG DSR 6 ----------------------- 20 DTR CTS 8 -| RTS 7 8 DCD RI 9 22 RI |
Cable Diagram 4.a
Uses: Calcomp Digitiser
Computer End Device End No Pins: 25 No Pins: 25 Sex: Female Sex: Male PG 1 1 PG TD 2 ----------------------- 2 TD RD 3 ----------------------- 3 RD RTS 4 4 RTS CTS 5 5 CTS DSR 6 6 DSR SG 7 ----------------------- 7 SG DCD 8 8 DCD DTR 20 20 DTR RI 22 22 RI |
Note:Although this may seem a simple cable it is infact all that is required to achieve communications. See Diagram 4.b for a 9 to 25 pin version of this cable.
Cable Diagram 4.b
Uses: 9 to 25 pin version of Diagram 4.a
Computer End Device End No Pins: 9 No Pins: 25 Sex: Female Sex: Male DCD 1 1 PG RD 2 ----------------------- 3 RD TD 3 ----------------------- 2 TD DTR 4 4 RTS SG 5 ----------------------- 7 SG DSR 6 5 CTS RTS 7 6 DSR CTS 8 8 DCD RI 9 20 DTR 22 RI |
Cable Diagram 5.a
Uses: OCE Digitisers
Note:With the exception of the pin 2 and pin 3 crossover this would be a modem cable. See Diagram 5.b for a 9 to 25 pin version of this cable.
Computer End Device End No Pins: 25 No Pins: 25 Sex: Female Sex: Male PG 1 ----------------------- 1 PG TD 2 ----------------------- 3 RD RD 3 ----------------------- 2 TD RTS 4 ----------------------- 4 RTS CTS 5 ----------------------- 5 CTS DSR 6 ----------------------- 6 DSR SG 7 ----------------------- 7 SG DCD 8 ----------------------- 8 DCD DTR 20 ----------------------- 20 DTR RI 22 22 RI |
Cable Diagram 5.b
Uses: 9 to 25 pin version of Diagram 5.a
Computer End Device End No Pins: 9 No Pins: 25 Sex: Female Sex: Male 1 PG DCD 1 ---------------------- 8 DCD RD 2 ---------------------- 2 TD TD 3 ---------------------- 3 RD DTR 4 ---------------------- 20 DTR SG 5 ---------------------- 7 SG DSR 6 ---------------------- 6 DSR RTS 7 ---------------------- 4 RTS CTS 8 ---------------------- 5 CTS RI 9 ---------------------- 22 RI |