RS-232 vs RS-485: What Is the Difference?

RS-232 and RS-485 are both serial standards that send data one bit at a time, but they solve different problems. RS-232 links exactly two devices over a short cable, with each signal measured against a shared ground. RS-485 sends each signal as the difference between two twisted wires, which cancels noise, reaches around 1,200 m and lets many devices share one pair of wires as a bus. RS-232 is the classic PC serial port; RS-485 is the usual choice for linking meters, drives and controllers across a plant.

Distances and speeds trade against each other on both standards. The figures are typical for good cable and should be checked against the device datasheets.

On RS-232, each signal is a voltage on one wire measured against the ground wire. Noise picked up along the cable, or a difference in ground potential between the two ends, shifts that voltage directly, so the link only works over short, clean runs. Our explainer on how RS-232 works covers its pins and handshaking.

RS-485 sends each signal on a pair of wires driven in opposite directions. The receiver looks only at the difference between them. Interference that hits both wires equally is ignored, and the receiver can detect a difference as small as 200 mV, so the signal survives long cables near motors and drives. Twisting the pair keeps the pickup equal on both wires.

RS-232 is strictly point to point. RS-485 allows many devices on one cable, which is why it underpins so many fieldbuses. Getting the wiring right matters more than with RS-232:

RS-422 uses the same differential signaling as RS-485 and reaches the same distance, but only one transmitter is allowed, driving up to 10 receivers on separate send and receive pairs. It suits long full-duplex links between two devices, or one controller broadcasting to several displays. Most RS-485 transceivers can also talk to RS-422 devices when wired for four wires.

Neither standard is new, but both remain common because the equipment they connect lasts for decades. RS-232 survives on lab instruments, weighing scales, GPS receivers, UPS management ports and the console ports of network switches and routers, where a short cable to one device is all that is needed. RS-485 survives because it is cheap, robust and well understood: energy meters, variable-speed drives, building management controllers, PTZ cameras and lighting desks still ship with it, and a single pair of wires can link dozens of them across a site. Replacing a working serial network with Ethernet is rarely worth the rewiring.

Match the standard to the far-end device and the cable run. Use RS-232 for short links to instruments, scales, barcode readers and console ports. Use RS-485 for long runs, noisy areas and any bus with several devices, such as Modbus meters or PTZ camera control. Many industrial computers have serial ports that switch between RS-232, RS-422 and RS-485 in the BIOS or with jumpers, so one system can handle a mixed site.

Isolation is worth specifying on RS-485 ports that run between buildings or cabinets, because ground differences and surges travel along the cable. Where a computer needs more ports, our serial I/O modules add opto-isolated RS-232/422/485 ports on mPCIe or PC/104. The British Steel case study shows protocol-selectable serial on rugged panel PCs.

We fit RS-232/422/485 ports and isolated serial modules to industrial computers as standard, set the port modes and test each one before the system ships. Tell us the devices, protocols and cable runs, and we will specify the serial ports to match.