25 PIN D-SUB MALE to Computer
The IBM PC parallel port (IEEE 1284-A, DB-25) exposes eight data output lines, four control outputs (STROBE*, AUTOFD*, INIT*, SELECTIN* on pins 1, 14, 16, and 17), and five status inputs (ACK*, BUSY, PError, SELECT, and ERROR* on pins 10–13 and 15). This passive loopback plug shorts those four control outputs back to four status inputs, plus one data line to the remaining status input, enabling diagnostic software such as CheckIt, AMI Diagnostics, and PC-Doctor to verify port operation without external hardware. The plug uses a DB-25 male connector that seats directly on the PC’s parallel-port socket; no active electronics are required.
| Name | Pin | Pin | Name |
|---|---|---|---|
| Busy | 11 | 17 | Select Input |
| Acknowledge | 10 | 16 | Initialize |
| Paper end | 12 | 14 | Auto Feed |
| Select | 13 | 1 | Strobe |
| Data Bit 0 | 2 | 15 | Error |
Signals
- Busy (pin 11) — status input; driven low by a peripheral to signal it is ready; shorted to SELECTIN* output (pin 17)
- Acknowledge (ACK*, pin 10) — status input; active-low acknowledge from peripheral; shorted to INIT* output (pin 16)
- Paper end (PError, pin 12) — status input; high when peripheral reports paper-out or fault; shorted to AUTOFD* output (pin 14)
- Select (pin 13) — status input; high when peripheral is on-line; shorted to STROBE* output (pin 1)
- Data Bit 0 (pin 2) — data output; shorted to ERROR* status input (pin 15)
Notes
- The connector is a DB-25 male, matching the female socket fitted to IBM-compatible PC parallel ports; the plug seats directly without a cable.
- Five connections are made: 1↔13, 2↔15, 10↔16, 11↔17, and 12↔14.
- This wiring is also used by AMI Diagnostics and PC-Doctor; it differs from the Norton/PassMark variant, which routes data lines (pins 2–6) to status inputs instead of control outputs.
- Pins 3–9 (Data Bits 1–7) and the ground lines (pins 18–25) are left unconnected.
References
- Serial and Parallel Port External Loopback Wiring Diagrams — PC-Doctor, Inc.
- Serial & Parallel Diagnostic Loopback Plugs, Revision 1.01 — David D. Scribner
- Application Note 062: IEEE 1284 – Updating the PC Parallel Port — National Instruments Corporation