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Phone: +86 16626708626Description
- Key Technical Specifications
| Parameter | Specification |
| Standard Specification | EIA/TIA-232-C (adopted by IEEE / ITU-T V.24) |
| Signal Topology | Point-to-point, full-duplex asynchronous serial |
| Logic Levels (Data) | Mark (Logic 0): +3 V to +15 V DC | Space (Logic 1): -3 V to -15 V DC |
| Logic Levels (Control) | Asserted (ON): +3 V to +15 V DC | De-asserted (OFF): -3 V to -15 V DC |
| Maximum Data Rate | 20 kbps (standard spec; up to 115.2 kbps over short low-capacitance runs) |
| Maximum Cable Length | 15 m (50 ft) or total capacitance limit < 2,500 pF |
| Connector Types | DB9 (DE-9) or DB25 male/female connectors |
| Pinout Assignment (DB9) | Pin 2: RxD, Pin 3: TxD, Pin 5: GND, Pin 7: RTS, Pin 8: CTS |
| Operating Temperature | -40 to +85°C (-40 to +185°F) |
| Shielding | Foil + braided copper shield with integrated ground wire |
- Product Introduction
The RS-232C is the industry-standard physical interface specification for asynchronous point-to-point serial communication between Data Terminal Equipment (DTE, like industrial PCs and PLCs) and Data Circuit-terminating Equipment (DCE, like modems and programming adapters). Defined by the EIA in 1969, revision C established the voltage threshold limits and signal timing guidelines that remain foundational in legacy process control systems today.
Utilizing bi-polar voltage levels (+3 V to +15 V for space/ON and -3 V to -15 V for mark/OFF) referenced to a common signal ground, the RS-232C interface provides reliable, full-duplex serial data transmission over unshielded or shielded multi-conductor cables. Plant engineers and technicians rely on high-grade RS-232C assemblies for diagnostic serial interfaces, drive parameter configuration ports, barcode reader interfaces, and legacy HMI communications.
RS-232C
- Troubleshooting Quick Reference
| Symptom | Possible Cause | Relevance to this Part | Quick Check Method | Recommendation |
| Complete communication failure (No data transmitted) | TxD and RxD lines crossed or straight-through cable used instead of Null Modem | ✅ High | Measure idle DC voltage on Pin 2 and Pin 3 relative to Pin 5 (GND) | Idle state must be negative (-5 V to -12 V DC). If both TxD lines are wired together, voltage drops to ~0 V DC. Install a Null Modem cross-over adapter. |
| Framing errors / Garbage characters in terminal software | Baud rate, parity, or stop bit settings mismatch between nodes | ❌ Low | Connect serial analyzer tool or verify serial port COM settings in OS | Match baud rate, data bits (8), parity (None/Even/Odd), and stop bits (1 or 2) across both communicating devices. |
| Intermittent data dropouts / Buffer overruns | Missing hardware flow control (RTS/CTS) or excessive line noise | ✅ High | Monitor RTS (Pin 7) and CTS (Pin 8) line toggle states during transfer | Enable XON/XOFF software handshaking if hardware RTS/CTS wires are not present in the cable harness. |
| Serial port unresponsive after electrostatic event | Ground loop potential or ESD strike on RS-232 driver chip | ✅ High | Measure resistance between signal ground (Pin 5) and protective earth | Install optically isolated RS-232 isolators when connecting equipment located across different electrical panel grounds. |
- Frequently Asked Questions (FAQ)
Q: What is the difference between a Straight-Through cable and a Null Modem cable?
A: A Straight-Through cable connects pin 2 to pin 2 and pin 3 to pin 3 (used to connect a DTE controller to a DCE modem). A Null Modem cable crosses pin 2 (RxD) and pin 3 (TxD) so that the transmit line of the host connects directly to the receive line of the target (used to connect two DTE devices directly, such as a PC to a PLC or drive).
Q: Can I extend an RS-232C run beyond the 50-foot (15-meter) specification limit?
A: The official EIA specification limits distance based on a maximum total line capacitance of 2,500 pF. By using specialized low-capacitance shielded cabling (e.g., Belden 9922), you can extend low-baud-rate (9600 bps) runs up to 100-200 feet. However, for longer industrial distances, converting RS-232 to RS-422 or RS-485 differential signals is strongly recommended.
Q: Why do I need a USB-to-RS232 converter with a FTDI chipset for programming PLCs?
A: Modern laptop USB ports do not have native UART hardware. Cheaper USB-to-Serial chips often drop handshaking lines or fail to generate true ±9 V DC bipolar logic levels required by older PLC serial ports. FTDI and Prolific industrial-grade converters generate stable voltage swings and correctly handle hardware handshaking lines (RTS/CTS/DTR/DSR).
Q: What is the purpose of the ground shield in high-noise factory environments?
A: Unshielded serial cables act as antennas for high-frequency electromagnetic interference (EMI) generated by VFDs and large contactors. High-grade RS-232C cables use a 100% foil shield plus tinned copper braid connected to protective earth at one chassis end to keep ground loop currents out of signal ground (Pin 5).
Q: How do you quality test your RS-232C cable assemblies?
A: Every serial cable assembly passes a 5-step SOP inspection. We perform 100% pinout continuity checking, high-voltage insulation testing between adjacent conductors (>10 MΩ at 500 V DC), flex strain relief testing, and a live serial bit-error-rate test (BERT) at 115.2 kbps before dispatch.
