The standard Hayes-compatible modem cable connects a PC serial port (Data Terminal Equipment, DTE) to an external modem (Data Circuit-Terminating Equipment, DCE) using the RS-232 interface standardised as ANSI/TIA/EIA-232-F (1997), formerly known as RS-232-C. The standard, first introduced in 1962, aligns with ITU-T V.24, V.28, and ISO/IEC 2110. PC serial ports use a 9-pin DE-9 (DB9S) female connector, while the modem end traditionally uses a 25-pin DB-25 male connector as specified in the original standard. Both cable variants described here are straight-through: each signal pin on the DTE side connects directly to its corresponding pin on the DCE side, passing all handshaking lines — TD, RD, RTS, CTS, DSR, DCD, DTR, and RI — without crossing.
9 pin D-SUB female connector to the Computer
25 pin D-SUB male connector to the modem
Modem DSUB9 to DSUB25 cable
| Female | Male | Dir |
|---|---|---|
| Shield | 1 | |
| Transmit Data | 3 | 2 |
| Receive Data | 2 | 3 |
| Request to Send | 7 | 4 |
| Clear to Send | 8 | 5 |
| Data Set Ready | 6 | 6 |
| System Ground | 5 | 7 |
| Carrier Detect | 1 | 8 |
| Data Terminal Ready | 4 | 20 |
| Ring Indicator | 9 | 22 |
Modem DSUB25 to DSUB25 cable
| Female | Male | Dir |
|---|---|---|
| Shield Ground | 1 | 1 |
| Transmit Data | 2 | 2 |
| Receive Data | 3 | 3 |
| Request to Send | 4 | 4 |
| Clear to Send | 5 | 5 |
| Data Set Ready | 6 | 6 |
| System Ground | 7 | 7 |
| Carrier Detect | 8 | 8 |
| Data Terminal Ready | 20 | 20 |
| Ring Indicator | 22 | 22 |
Notes
Both cable variants are straight-through (pin-for-pin); no signal crossing is required because the DTE and DCE roles are complementary by design under TIA/EIA-232-F. The DE-9 female connector is fitted to the DTE (computer) end; the DB-25 male connector is fitted to the DCE (modem) end. TIA/EIA-232-F specifies a maximum cable capacitance of 2500 pF (typically 15–20 m) and a maximum data signalling rate of 20 kbps, though many implementations operate at higher rates. Driver output must be at least ±5 V under load; receiver sensitivity is ±3 V minimum. The DE-9 pin assignments for DTE are: pin 1 DCD, pin 2 RD, pin 3 TD, pin 4 DTR, pin 5 GND, pin 6 DSR, pin 7 RTS, pin 8 CTS, pin 9 RI.
Signals
- Shield / Shield Ground — chassis or cable-shield ground; pin 1 on both DE-9 and DB-25; the DE-9 shield pin has no corresponding modem signal and is typically left unconnected or tied to chassis at the modem end
- Transmit Data (TD) — serial data from DTE to DCE; DE-9 pin 3 to DB-25 pin 2
- Receive Data (RD) — serial data from DCE to DTE; DE-9 pin 2 to DB-25 pin 3
- Request to Send (RTS) — DTE requests permission to transmit; DE-9 pin 7 to DB-25 pin 4
- Clear to Send (CTS) — DCE grants permission to transmit; DE-9 pin 8 to DB-25 pin 5
- Data Set Ready (DSR) — DCE indicates it is ready and connected to line; DE-9 pin 6 to DB-25 pin 6
- System Ground (GND) — signal reference ground; DE-9 pin 5 to DB-25 pin 7
- Carrier Detect (DCD) — DCE indicates a carrier has been detected on the line; DE-9 pin 1 to DB-25 pin 8
- Data Terminal Ready (DTR) — DTE signals it is ready to communicate; DE-9 pin 4 to DB-25 pin 20
- Ring Indicator (RI) — DCE signals that an incoming ring is detected; DE-9 pin 9 to DB-25 pin 22
References
- Low-Voltage, Single-Supply 232-Standard Interface Solutions (SLLA083A) — Texas Instruments, Incorporated