The LapLink/INTERLNK parallel cable is a DB-25 male-to-DB-25 male null-printer cable that connects two PCs via their parallel (LPT) ports for direct file transfer. It exploits the original IBM PC parallel printer adapter design, which provides five unidirectional data outputs (pins 2–6) and five unidirectional status inputs (pins 10–13 and 15), by cross-wiring each side’s outputs to the other side’s inputs. This achieves half-duplex bidirectional transfer without requiring a bidirectional parallel port, with software reading five-bit nibbles and reassembling bytes. MS-DOS 6.0–6.22 INTERLNK/INTERSVR, Traveling Software LapLink (from 1983), Windows 95 Direct Cable Connection, and Norton Commander v4.0 and v5.0 all support this cable.
Will work with:
- LapLink from Travelling Software
- MS-DOS v6.0 InterLink from Microsoft
- Windows 95 Direct Cable connection from Microsoft
- Norton Commander v4.0 & v5.0 from Symantec
25 PIN D-SUB MALE to Computer 1
25 PIN D-SUB MALE to Computer 2
| Name | Pin | Pin | Name |
|---|---|---|---|
| Data Bit 0 | 2 | 15 | Error |
| Data Bit 1 | 3 | 13 | Select |
| Data Bit 2 | 4 | 12 | Paper Out |
| Data Bit 3 | 5 | 10 | Acknowledge |
| Data Bit 4 | 6 | 11 | Busy |
| Acknowledge | 10 | 5 | Data Bit 3 |
| Busy | 11 | 6 | Data Bit 4 |
| Paper Out | 12 | 4 | Data Bit 2 |
| Select | 13 | 3 | Data Bit 1 |
| Error | 15 | 2 | Data Bit 0 |
| Reset | 16 | 16 | Reset |
| Select | 17 | 17 | Select |
| Signal Ground | 18-25 | 18-25 | Signal Ground |
Notes
The cross-wiring pattern is symmetrical: pin 2↔15, 3↔13, 4↔12, 5↔10, 6↔11 (mirrored on both sides), with ground pins 18–25 straight-through. The minimal INTERLNK-only cable documented by Microsoft requires exactly 11 wires — pins 2–6, 10–13, 15, and 25 — and omits pins 16 and 17. The table above reflects the broader commercial LapLink cable variant, which additionally connects the control lines Reset (pin 16) and SelectIn (pin 17) straight-through for improved handshaking compatibility. All signals operate at TTL levels (0–5 V); logic HIGH is approximately 2.4–5.0 V and logic LOW is 0–0.8 V, with rise and fall times under 0.2 µs per the IBM PC parallel port specification.
Signals
- Data Bit 0–4 (pins 2–6) — unidirectional output data lines from the parallel port; active High; cross-connected to the status inputs of the remote port
- Acknowledge (pin 10) — status input; active Low; receives Data Bit 3 from the remote port
- Busy (pin 11) — status input; active High; receives Data Bit 4 from the remote port
- Paper Out (pin 12) — status input; active High; receives Data Bit 2 from the remote port
- Select (pin 13) — status input; active High; receives Data Bit 1 from the remote port
- Error (pin 15) — status input; active Low; receives Data Bit 0 from the remote port
- Reset (pin 16) — control line; straight-through connection; used by the extended LapLink cable variant for initialisation handshaking
- Select (pin 17) — control line (SelectIn); straight-through connection; used by the extended LapLink cable variant
- Signal Ground (pins 18–25) — common ground reference; pin 25 is the minimum ground connection required by the INTERLNK specification