The LapLink serial cable is a full-handshake null modem cable using DE-9 (9-pin D-sub) connectors at both ends, designed for direct computer-to-computer communication without an intermediate modem. It was popularised by Travelling Software’s LapLink product and is also compatible with the MS-DOS INTERLNK/INTERSVR utilities. The cable crosses transmit and receive data lines, Data Terminal Ready with Data Set Ready, and Request To Send with Clear To Send, satisfying software that monitors all hardware handshake lines. Null modem wiring of this type is not defined by TIA-232 or ITU-T V.24; several incompatible variants exist in practice.
Will work with:
- LapLink from Travelling Software
9 PIN D-SUB FEMALE to Computer 1
9 PIN D-SUB FEMALE to Computer 2
| Name | Pin | Pin | Name |
|---|---|---|---|
| Receive Data | 2 | 3 | Transmit Data |
| Transmit Data | 3 | 2 | Receive Data |
| Data Terminal Ready | 4 | 6 | Data Set Ready |
| Signal Ground | 5 | 5 | Signal Ground |
| Data Set Ready | 6 | 4 | Data Terminal Ready |
| Request To Send | 7 | 8 | Clear To Send |
| Clear To Send | 8 | 7 | Request To Send |
Signals
- RxD — Receive Data (ITU-T V.24 circuit 104); data received by the DTE from the remote machine’s TxD.
- TxD — Transmit Data (ITU-T V.24 circuit 103); data sent by the DTE to the remote machine’s RxD.
- DTR — Data Terminal Ready (ITU-T V.24 circuit 108); indicates the local DTE is powered and ready; crosses to the remote DSR.
- GND — Signal Ground (ITU-T V.24 circuit 102); common reference for all signals.
- DSR — Data Set Ready (ITU-T V.24 circuit 107); indicates the remote DTE is ready; driven by the remote DTR.
- RTS — Request To Send (ITU-T V.24 circuit 105); flow-control output from the local DTE; crosses to the remote CTS.
- CTS — Clear To Send (ITU-T V.24 circuit 106); flow-control input to the local DTE; driven by the remote RTS.
Notes
- The cable uses DE-9 female connectors at both ends (9-pin D-sub, per TIA-574).
- Pin 1 (DCD) and pin 9 (RI) are not connected in this cable.
- Crossing pattern: TxD↔RxD, DTR↔DSR, RTS↔CTS — all end-to-end crossed, Signal Ground straight through.
- This is a full-handshake null modem variant; it satisfies software that polls DSR and CTS before transmitting.
- Maximum recommended cable length under RS-232/V.28 electrical rules is 15 m (50 ft) or 2500 pF total capacitance at the rated speed.
- Null modem wiring is not standardised by TIA-232 or ITU-T V.24; incompatible variants (partial-handshake, loopback-handshake, no-handshake) exist and may not interoperate with LapLink software.