A serial loopback plug (also called a wrap plug) is a diagnostic connector that shorts a serial port’s transmit outputs back to its receive inputs, allowing diagnostic software to verify UART hardware without an external device. The RS-232 standard governing these signals is ANSI/TIA-232-F; the 9-pin DE-9 connector form was introduced on the IBM PC/AT in 1984 and standardised under TIA-574. This female DE-9 variant is designed for use with Eurosoft PC-Check diagnostic software and uses a distinct pin grouping that differs from the PassMark and Norton loopback schemes.
9 pin D-SUB female connector
| Pin | Name | Description |
|---|---|---|
| 1 | 1-4-9 | |
| 2 | 2-3 | |
| 3 | 6-7-8 | |
Signals
- 1-4-9 — DCD (Data Carrier Detect), DTR (Data Terminal Ready), and RI (Ring Indicator) shorted together
- 2-3 — TxD (Transmit Data) and RxD (Receive Data) shorted together
- 6-7-8 — DSR (Data Set Ready), RTS (Request To Send), and CTS (Clear To Send) shorted together
Notes
The plug mates with the male DE-9 serial port on the rear panel of a PC; no cabling is required. This grouping — pins 1, 4, and 9 linked together and pins 6, 7, and 8 linked together — is specific to the PC-Check variant and differs from the PassMark/PC-Doctor scheme (which links pins 1-4-6 and omits RI) and the Norton/CheckIt scheme (which links pins 1-4-6-9). The RS-232 standard specifies that DTE ports use male connectors, so the loopback plug uses a female connector to mate with the PC port.
References
- Serial (RS232) and Parallel Loopback Plugs Technical Specifications — PassMark Software
- SunVTS 6.2 Test Reference Manual for x86 Platforms — Appendix A: Loopback Connectors — Oracle Corporation
- Serial and Parallel Port External Loopback Wiring Diagrams — PC-Doctor, Inc.