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RS232 Wiring Diagrams
By A.P. Lawrence
Expert Author
Article Date: 2003-03-27
Meanings of pins in DB25 order:
| DB25 Pin | DB9 Pin | Description | | 1 | | FG Frame Ground | | 2 | 3 | TD Transmit data (Out) | | 3 | 2 | RD Receive data (In) | | 4 | 7 | RTS Request to Send (Out) | | 5 | 8 | CTS Clear to Send (In) | | 6 | 6 | DSR Data Set Ready (In) | | 7 | 5 | SG Signal Ground | | 8 | 1 | DCD Data Carrier Detect (In) | | 9 | | +V (In) | | 10 | | -V (In) | | 11 | | QM (also called SSD- Secondary Send Data)(In) | | 12 | | SDCD Secondary Carrier Detect (In) | | 13 | | SCTS Secondary Clear to Send (In) | | 14 | | STD Secondary Transmit Data (Out) | | 15 | | TC Transmitter Clock (In) | | 16 | | SRD Secondary Receive Data (In) |
17 | | RC Receiver Clock (In) | | 18 | | Unused | | 19 | | SRTS Secondary Request to Send (Out) | | 20 | 4 | DTR Data Terminal Ready (Out) | | 21 | | SQ Signal Quality Detect (In) | | 22 | 9 | RI Ring Indicator (In) | | 23 | | Data Rate Select (Out) | | 24 | | (TC) External Transmitter Clock (Out) | | 25 | | Unused |
Meanings of pins in DB9 order:
| DB25 Pin | DB9 Pin | Description | | 8 | 1 | DCD Data Carrier Detect (In) | | 3 | 2 | RD Receive data (In) | | 2 | 3 | TD Transmit data (Out) | | 20 | 4 | DTR Data Terminal Ready (Out) | | 7 | 5 | SG Signal Ground | | 6 | 6 | DSR Data Set Ready (In) | | 4 | 7 | RTS Request to Send (Out) | | 5 | 8 | CTS Clear to Send (In) | | 22 | 9 | RI Ring Indicator (In) |
Connecting to other devices
The unfortunate thing about RS232 is that aside from the simplest TD to RD and SG to SG hookups, and computer to modems (essentially straight through because a modem is DCE- just the reverse of a computer), everybody does it differently. For the diagrams below, leave off FG when you don't have it (DB9).
Async modems need FG (if available), SG, TD, RD, CTS, RTS, DSR, DCD and DTR at a minimum. Synchronous modems also need SCTS, STD, TC, RC, RI and (TC).
There is tremendous variance in printer wiring. If you don't know how your printer expects to be wired, you'll have to wire it just TD,RD and SG, and rely on Xon/Xiff flow control.
Some printers handshake with RTS and use DTR to indicate on-line or off-line.
| Computer | Printer (handshake DTR and CTS) | | FG | FG | | RD | TD | | TD | RD | | DCD | RTS | | RTS | DCD | | CTS and DSR | DTR | | DTR | CTS and DSR | | SG | SG |
Others use DTR for everything
| Computer | Printer (handshake DTR) | | FG | FG | | RD | TD | | TD | RD | | CTS, DCD and DSR | DTR | | DTR | CTS, DCD and DSR | | SG | SG |
Okidata printers have an unusual setup:
| Computer | Okidata Serial Printer | | FG | FG | | RD | TD | | TD | RD | | CTS | QM | | DSR | DTR and DSR | | SG | SG |
Connecting a computer to another computer needs only TD, RD and SG if you are using Xon/Xoff flow control.
| Computer | Terminal or Computer | | FG | FG | | RD | TD | | TD | RD | | RTS | CTS | | CTS | RTS | | DCD and DSR | DTR | | DTR | DCD and DSR |
| SG | SG |
See also Serial Wiring and Serial Printers
About the Author:
A.P. Lawrence provides SCO Unix and Linux consulting services http://www.pcunix.com
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