Definition
RS232 is a point-to-point serial communication standard, using ±12V levels, with a transmission distance usually not exceeding 15 meters. Industrial equipment commonly uses RS232 to connect the debug port, printer, or a single instrument. When networking multiple devices, RS485 is usually used.
Technical Features
RS232 uses single-ended signals, with a three-wire system of TX, RX, and GND (optionally with RTS and CTS flow control). Voltage levels are ±5V to ±15V, incompatible with TTL, requiring level conversion. Typical baud rates are 9600, 19200, 115200, etc. Transmission distance is short (within 15m), with weak anti-interference capability, and it does not support multi-device buses. Industrial DTUs and gateways often provide RS232 interfaces for connecting a single device or debugging.
Use Cases
PLC and instrument debugging ports; printer, barcode scanner; single device point-to-point communication. For multiple devices and long-distance scenarios, RS485 should be used. RS232 to USB adapters are commonly used for industrial PCs and laptops to connect to serial devices. Industrial gateways can upload RS232 data to the cloud via 4G or Ethernet.
Single-Ended Levels and a Shared Ground
Negative logic, single-ended, and a shared ground is mandatory
This interface represents a bit with one signal wire measured against a common ground. The logic states are voltages on the order of plus or minus a dozen volts, and the logic is inverted: a negative voltage often means logic 1, and a positive voltage means logic 0. The receiver samples by recognizing the polarity of that wire relative to ground, so the two grounds have to be tied together. Otherwise the receiver sees a floating voltage. A waveform that looks clean still does not decode into stable bits. There is no differential pair to reject interference. A nearby motor or switch-mode supply lands directly on this wire.
A full handshake may also include request to send and clear to send. If the device requires hardware handshake and the cable connects only receive, transmit, and ground, the sender waits forever for the permit signal. The port opens and not a single byte goes out. Some devices treat a floating handshake pin as permitted by default. The same cable on another device does the opposite. The levels are not compatible with a microcontroller's 3.3 V or 5 V. A direct connection damages the port. A level translator has to sit in between.
One nearby device only
It suits only one device talking to one other device, with a short cable and a reliable common ground: a debug port, a nearby printer, a barcode scanner, or a single instrument. Distance should usually stay within about ten to twenty meters, and a higher rate means a shorter cable. Do not branch a second device, and do not "listen" by wiring in parallel. When ground potential already differs between cabinets, even a short run puts current on the common ground and misreads bits. In that case change to a differential bus or isolate first. Do not try to get by with a thicker ground wire.
Every parameter has to match: rate, data bits, parity, and stop bits. One of them different, and the receiver misreads the frame boundary. What prints is stable garbage, not random noise. Flow control has to match as well. Many industrial protocols can run on this link, including Modbus over a short distance, but the physical-layer limits do not disappear because a protocol was substituted.
Levels, pins, and ground loops
Pinout is the first class of field error. Some devices arrange transmit and receive as data terminal equipment, others as data circuit-terminating equipment. Both ends then transmit into each other. A meter measures a voltage, and there is no communication at all. Choose crossover or straight-through by device type. Do not go by cable color. Second, ground is not connected. With only two signal wires, the return travels through the chassis and protective earth. When the potential difference is large, communication comes and goes, and in a severe case the interface chip is destroyed.
Third, a long cable is used together with a very high rate. Edges slow down on the cable capacitance, the sample point falls in the transition, a local loopback works, and the real cable fails. Lower the rate before investigating the protocol. If neither side of an isolated converter is powered, the isolated side has no energy and the port is dead. Finally, do not hang several instruments on this single-ended interface. A correct address still does not create a multi-drop electrical capability. Forcing them in parallel only makes the drivers fight each other's voltage.
The direction of converter power is often ignored as well. A passive converter takes its power from the interface. On a long cable, with a weak driver at the far end, the swing is not enough. A local loopback works, and the same link on site produces a fixed wrong character. Confirm whether the converter needs an external supply, and look at the swing at both ends of the real cable on a scope to see that it still crosses the receive threshold. If the converter shorts chassis ground to signal ground, the ground loop that isolation was meant to break is connected again.
A ground potential difference at the two ends of a long cable can also push the interface chip into latch-up. Once leakage current flows in the common ground, the receiver's negative-voltage region is lifted, logic 1 is read as 0, and the frame is wrong in the same few bits every time. Measure the AC and DC voltage between the two grounds first, then decide whether to shorten the run, add an isolator, or change to a differential interface. Do not keep changing the baud rate.