RS-232

What is RS-232?

RS-232 (Recommended Standard 232) is a standard for serial, point-to-point data communication that defines the electrical signal levels, signal lines and connectors with which two devices, such as a computer and a PLC, HMI, modem or measuring instrument, exchange data bit by bit over a short distance. The Electronic Industries Association (EIA) introduced the standard in 1960 to connect terminals to modems. More than sixty years later, an RS-232 port still sits on countless sensors, weighing scales, barcode scanners, CNC machines and network devices. Anyone working in operational technology will run into RS-232 sooner or later, often on legacy systems that stay in service for decades.

🧠 What exactly does RS-232 do?

RS-232 describes only the physical layer: voltages, timing requirements, signal names and connectors. What the bits mean is decided by a protocol running on top of it, for example Modbus RTU, DNP3 or a vendor-specific ASCII protocol.

• Two roles - the standard distinguishes DTE (Data Terminal Equipment, such as a PC or terminal) and DCE (Data Circuit-terminating Equipment, originally the modem). Depending on the vendor, a PLC may be either DTE or DCE; the device documentation should state which.
• Separate transmit and receive lines - TXD and RXD run on separate conductors with a common signal ground, so both sides can transmit at the same time (full duplex).
• Voltage referenced to ground - RS-232 is single-ended: the receiver measures the voltage of one wire relative to signal ground. A voltage between +3 and +15 V means logic 0 (space), between −3 and −15 V logic 1 (mark). The region between −3 and +3 V is invalid. For the control lines the polarity is reversed: positive means “asserted”.
• Asynchronous frame - in practice transmission is almost always asynchronous: a start bit, 5 to 8 data bits, an optional parity bit and 1 or 2 stop bits. The standard also defines clock lines for synchronous modems, but these are rarely used in industry.

🕰️ How has the RS-232 standard evolved?

Internationally, the same rules are laid down in ITU-T V.24 (definition of the interchange circuits) and V.28 (electrical characteristics). The 25-pin DB-25 was the usual connector for decades and was formally written into the standard in 1986; the familiar 9-pin DE-9 (often loosely called “DB9”) arrived with the serial adapter card of the IBM PC-AT and was later standardised in TIA-574.

🔧 What are the technical characteristics of RS-232?

Distance is therefore set by the capacitance of the cable rather than a fixed number of metres. The Modbus serial line specification works it out: with a cable of 100 pF per metre you reach 25 metres, and it recommends RS-232 only for short links of typically less than 20 metres. At low baud rates and with good cable, longer runs sometimes work in practice, but that is outside the specification.

🧱 Which signal lines and pins does RS-232 use?

For most industrial links three conductors are enough: TXD, RXD and GND. When you connect two DTE devices (for example a laptop and an HMI), TXD and RXD must be crossed with a null modem cable. That cable is not part of the standard itself, and designs differ in how they loop back the handshake lines.

🏭 Where do you find RS-232 in industry?

• Configuration and programming - older PLCs, variable frequency drives and RTUs are programmed through a serial port.
• Console ports - switches, routers and firewalls have a serial console (often RJ45 or USB) for management and emergency recovery; 9600 bit/s, 8N1 is a common default.
• Instruments and peripherals - weighing scales, barcode scanners, label printers, analysers and energy meters send readings as ASCII lines.
• Machines - CNC controls have traditionally used RS-232 to load part programs (DNC).
• Protocols - Modbus RTU and DNP3 can run over RS-232; for several devices on one line, RS-485 is used instead.

🔄 What is the difference between RS-232 and RS-485?

That is why the Modbus specification recommends RS-485 for new serial designs and lists RS-232 only as an option for short point-to-point links. See fieldbuses for the wider picture.

🛠️ How do you set up an RS-232 link in practice?

1. Determine the roles - is each device DTE or DCE? That decides whether you need a straight-through or a null modem cable.
2. Match the parameters - baud rate, data bits, parity and stop bits must be identical on both sides. A common mistake: the PC is set to 9600 8N1, the device to 19200 8E1 (the Modbus default: 19,200 bit/s, even parity).
3. Choose the handshake - none, hardware (RTS/CTS) or software (XON/XOFF). A wrong handshake setting often produces a “dead” link.
4. Keep it short and mind the ground - keep the cable short, use shielded cable and avoid ground loops between cabinets. With EMC problems or different ground potentials, a galvanically isolated converter helps.
5. Test with a terminal program - a USB-to-serial adapter and a terminal program immediately show whether readable characters arrive.

🔐 How secure is RS-232?

RS-232 has no authentication and no encryption: anyone with physical access to the port can listen in or send commands. On its own, the short reach limits that risk, but the picture changes as soon as serial devices are connected to Ethernet through a serial device server or protocol gateway. Those converters suddenly make an old interface reachable over the network. Over the years CISA has published several ICS advisories on vulnerabilities in such serial device servers, ranging from denial of service to remote code execution.

Practical measures:

• Inventory serial links and converters in your asset inventory; they are easily forgotten.
• Segment converters into the right zone using network segmentation and the Purdue Model, and allow traffic only from known systems.
• Keep firmware up to date through patch management, disable unused web interfaces and Telnet, and change default passwords.
• Secure console access - route management through a jump server and controlled remote access, not through a modem or an open port.
• Physical security - lock cabinets, because a laptop with a serial cable bypasses every firewall. IEC 62443 treats such ports as part of the zone and conduit analysis.

❓ Frequently asked questions

What is the maximum cable length for RS-232?

The RS-232 standard does not specify a fixed cable length but a maximum cable capacitance of 2,500 pF. With typical cable this works out at roughly 15 to 25 metres. For longer distances, use RS-485 or a converter to Ethernet or fibre.

What does 9600 8N1 mean for RS-232?

9600 8N1 means 9600 bits per second, 8 data bits, no parity and 1 stop bit. Including the start bit, each RS-232 character then takes 10 bits, so about 960 characters per second. Both devices must use exactly the same setting.

What is the difference between DB9 and DB25 for RS-232?

Both are D-sub connectors for RS-232: the 25-pin DB-25 is the original connector, while the 9-pin DE-9 (often called “DB9”) carries only the most widely used signals. On a DE-9, TXD is on pin 3 and RXD on pin 2; on a DB-25 it is the other way round.

When do I need a null modem cable for RS-232?

You need a null modem cable when connecting two devices of the same type, for example two DTEs such as a laptop and an HMI. The cable crosses TXD and RXD so that one device’s transmit line arrives at the other device’s receive line.

Can I connect RS-232 directly to a microcontroller or TTL port?

No, RS-232 uses ±3 to ±15 V with inverted logic, whereas a TTL UART uses 0 to 3.3 or 5 V. Connecting them directly can damage the microcontroller. You need a level shifter, such as a MAX232-type chip or a ready-made module.

Is RS-232 still relevant for IT/OT convergence?

Yes, RS-232 remains relevant because many instruments, machines and network devices still have a serial port. In IT/OT integration these are connected to the network through gateways, so they must be included in the asset inventory, the segmentation design and the patching policy.

📌 In summary

RS-232 is a standard from 1960, now TIA-232-F, that connects two devices point to point using single-ended voltages between ±3 and ±15 V. It is simple and universal, but limited in distance, speed and scalability. For buses and longer distances RS-485 is the logical successor, and gateways bring serial devices onto Ethernet.

RS-232 is the simple serial point-to-point link still found in a great deal of OT equipment; keep the cable short, match the parameters exactly and, above all, secure the converters that put serial ports on the network.