Serial Standoff: How RS232 and RS485 Still Rule Separate Corners of the Shop Floor

On embedded benches and factory floors, RS232 and RS485 keep appearing side by side, yet they play nothing alike. Industry guides and vendor selection notes confirm the two are not a generational replacement pair, but parallel survivors chosen by distance, node count, and noise budget.

RS232 is among the oldest serial physical-layer standards. It uses single-ended signaling referenced to ground—logic 1 sits at -3V to -15V, logic 0 at +3V to +15V—runs full duplex, point to point, and stays reliable under roughly 15 meters with early-rate caps near 20kbps. That makes it the quiet default for a PC tuning a PLC, a GPS module, or a lab instrument panel.     

RS485 arrived to fix RS232’s weak spots. It drives a twisted pair A/B as differential lines, tolerates common-mode shift from -7V to +12V, shrugs off motor and VFD noise, and shares one bus half duplex across 32 to 256 nodes. At 9600bps it reaches 1200 meters; at short runs it clears 10Mbps. It is the de facto physical layer behind Modbus RTU, smart meters, and sensor drops.     

The news angle is boring on purpose: near, one-to-one, low noise → RS232; across bays, one master many slaves, heavy EMI → RS485. Even as Ethernet and USB creep onto the panel, RS485’s installed base in legacy automation is not going anywhere soon.