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 software to verify UART hardware without any external device. The 9-pin DE-9 form factor, standardised under TIA-574 and governed by the RS-232 electrical standard (ANSI/TIA/EIA-232-F), was introduced on the IBM PC/AT in 1984 and remains the most common serial port on desktop PCs. CheckIt, the diagnostic utility published by TouchStone Software (later Smith Micro), uses a loopback plug that loops the modem-control group including the Ring Indicator (RI) signal on pin 9, distinguishing it from plugs used by other diagnostic packages that omit RI. The female DE-9 plug connects directly to the male serial port on the PC’s rear panel with no cabling required.
9 PIN D-SUB FEMALE Connector
| Name | Pin | Pin | Name |
|---|---|---|---|
| CD | 1 | 6 | DSR |
| CD | 1 | 9 | RI |
| RXD | 2 | 3 | TXD |
| DTR | 4 | 6 | DSR |
| RTS | 7 | 8 | CTS |
Signals
- CD (Carrier Detect, pin 1) — looped to both DSR (pin 6) and RI (pin 9), forming a three-way modem-control group
- RXD (Receive Data, pin 2) — shorted to TXD so that data transmitted by the UART is immediately received back
- TXD (Transmit Data, pin 3) — looped to RXD (pin 2)
- DTR (Data Terminal Ready, pin 4) — looped to DSR (pin 6)
- DSR (Data Set Ready, pin 6) — receives both CD (pin 1) and DTR (pin 4)
- RTS (Request To Send, pin 7) — looped to CTS (pin 8) to verify hardware flow-control signalling
- CTS (Clear To Send, pin 8) — looped to RTS (pin 7)
- RI (Ring Indicator, pin 9) — looped to CD (pin 1); inclusion of RI distinguishes this plug from the PassMark and PC-Doctor variants
Notes
- The page title uses an alternative romanisation “CeckIt”; the product is correctly named CheckIt.
- The RS-232 standard designates DTE ports as male connectors; the loopback plug therefore uses a female DE-9 connector to mate with the PC’s male port.
- Pin 5 (Signal Ground) carries no loopback connection and is left unconnected in the plug.
- Loopback plugs must be seated firmly in the port for diagnostic tests to pass; a loose connection may cause intermittent test failures.
References
- Serial and Parallel Port External Loopback Wiring Diagrams — PC-Doctor, Inc.
- Serial (RS232) and Parallel Loopback Plugs Technical Specifications — PassMark Software
- SunVTS 6.2 Test Reference Manual — Appendix A: Loopback Connectors — Oracle Corporation