Serial connections: V.24 / RS-232, DB-25, DB-9 and VT100
Pinouts, cabling and terminal settings for the console and maintenance ports of legacy equipment.
Besides SCSI, many legacy telecom and industrial systems have a serial port used for the console, maintenance and data transfer. The standard is V.24 (ITU-T) / RS-232 (EIA), with signal levels defined by V.28. The connector is a 25-pin D-sub (DB-25) or the smaller 9-pin D-sub (DB-9).
A VT100 is the classic DEC text terminal. Most systems that expect a VT100 simply need a serial connection to a PC running a terminal program with VT100 emulation.
Signals and pinout
| Signal | V.24 circuit | DB-25 pin | DB-9 pin | Direction |
|---|---|---|---|---|
| TxD Transmit data | 103 | 2 | 3 | Out |
| RxD Receive data | 104 | 3 | 2 | In |
| RTS Request to send | 105 | 4 | 7 | Out |
| CTS Clear to send | 106 | 5 | 8 | In |
| DSR Data set ready | 107 | 6 | 6 | In |
| GND Signal ground | 102 | 7 | 5 | – |
| DCD Data carrier detect | 109 | 8 | 1 | In |
| DTR Data terminal ready | 108/2 | 20 | 4 | Out |
| RI Ring indicator | 125 | 22 | 9 | In |
Directions are seen from the DTE (terminal or PC). A DCE (modem) uses the opposite directions.
Null-modem cable (two DTE devices)
- Minimum, 3 wires: TxD to RxD, RxD to TxD, GND to GND.
- Full handshake: also cross RTS with CTS, and DTR with DSR and DCD.
- A straight-through cable is for DTE to DCE (PC to modem). Joining two computers, or a PC to a terminal port, usually needs a null-modem cable.
VT100 terminal
The VT100 is a DEC text terminal from 1978 that defined the ANSI escape codes still used by most terminal programs. Equipment that asks for a VT100 expects these codes.
Use any terminal program with VT100 emulation (on Unix-type systems, TERM=vt100).
Typical serial settings
| Baud rate | 9600 (often 1200 to 19200) |
|---|---|
| Data bits | 8 (older systems: 7) |
| Parity | None (older systems: even) |
| Stop bits | 1 |
| Flow control | None, or RTS/CTS |
Always check the equipment manual: settings differ between systems.
Practical tips
USB-to-serial adapters
Modern PCs have no serial port. A USB-to-RS-232 adapter with a good chipset works with most equipment.
Cable length
RS-232 is meant for short runs, typically up to about 15 m (50 ft). Use a lower baud rate for longer cables.
Connector gender
Check male and female connectors and use a gender changer if needed. Confirm whether the equipment is DTE or DCE before choosing the cable.
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