RS-485 vs RS-232 Serial Control: A Technical Reference

The short answer

RS-232 and RS-485 are both serial standards, but they solve different problems. RS-232 (EIA/TIA-232) is a single-ended, point-to-point link: one driver, one receiver, over one signal wire per direction referenced to a common ground. It swings between roughly +3 to +15 V and -3 to -15 V, uses inverted logic (a negative voltage is a logic 1/mark, a positive voltage is a logic 0/space), and is practical to about 15 m (50 ft) at modest baud rates. RS-485 (EIA/TIA-485, also TIA/EIA-485-A) is differential (balanced): it transmits each bit as the voltage difference between two conductors (A and B), which cancels common-mode noise and lets one bus reach up to about 1200 m (4000 ft) and connect many devices on the same pair.

The short decision rule: use RS-232 for a short one-to-one cable run between two nearby devices (a projector to a controller, a PC to a piece of equipment). Use RS-485 when you need distance, electrical noise immunity, or more than two devices on a shared line (a multidrop bus). RS-485 supports 32 standard unit loads on a segment, with 120-ohm termination at both physical ends of the bus. That is why access-control readers (OSDP), PTZ camera control (Pelco-D/P), Modbus RTU, and building-automation networks (BACnet MS/TP) almost always run on RS-485, while short device-to-device control links often stay on RS-232.

Which One You Actually Need

Start with three questions: how far, how many devices, and how noisy the environment. RS-232 (EIA/TIA-232) is a point-to-point link with exactly one driver and one receiver per signal, referenced to a shared ground. It is the correct choice for a short cable between two nearby devices, for example a control processor to a projector or display, a PC to a piece of test equipment, or a head-end to a single serially controlled device. Its practical reach is about 15 m (50 ft), and it has no built-in way to address multiple devices on one line.

RS-485 (EIA/TIA-485) is the answer whenever you need distance, electrical noise immunity, or a shared bus. Because it is differential, it tolerates the electrical noise found in commercial buildings and long conduit runs, and because it is multidrop, many devices can hang off the same twisted pair. A single segment can carry 32 standard unit loads and reach roughly 1200 m (4000 ft) at lower baud rates. That combination is why field buses for access control, camera control, HVAC, and metering standardized on RS-485 rather than RS-232.

A practical middle case is worth naming: many pieces of AV and control gear still expose an RS-232 port for local device control, while the network tying dozens of field devices together across a building is RS-485. It is common to see both in one system. When a run outgrows RS-232's length limit or needs more than two endpoints, an RS-232-to-RS-485 converter bridges the two without changing the higher-level protocol.

• Two devices, short cable, one-to-one: RS-232 is the simplest fit (about 15 m / 50 ft practical).
• Long run, multiple devices, or electrically noisy space: RS-485 (up to ~1200 m / 4000 ft, up to 32 unit loads).
• RS-232 has one driver and one receiver per signal; RS-485 is a multidrop bus on one twisted pair.
• RS-232 is single-ended (referenced to ground); RS-485 is differential (noise-canceling A/B pair).
• Mixed systems are normal: RS-232 for local device control, RS-485 for the building-wide field bus.
• Bridge the two with an RS-232-to-RS-485 converter when a run exceeds RS-232's length or endpoint limits.

Voltage Levels and Signaling

RS-232 is single-ended: each signal is one wire whose voltage is measured against a common ground. Drivers output between +5 and +15 V and between -5 and -15 V (the standard permits swings up to +/-15 V), and the region between -3 V and +3 V is undefined. The logic is inverted relative to what most people expect: a negative voltage is a logic 1, called a mark, and a positive voltage is a logic 0, called a space. Receivers are specified with +/-3 V input sensitivity and must tolerate inputs up to +/-15 V. Because the signal rides on a single conductor against ground, any noise coupled onto the line is added directly to the signal.

RS-485 is differential (balanced): each bit is carried as the voltage difference between two conductors, commonly labeled A and B (or - and +). A compliant driver produces a differential output of at least 1.5 V under load, and a compliant receiver reliably detects a differential input as small as +/-200 mV. The receiver reads greater than +200 mV as one logic state and less than -200 mV as the other, and it must do so across a common-mode range of -7 V to +12 V. That large margin between the driver's output and the receiver's threshold, combined with noise appearing almost equally on both wires and canceling at the receiver, is what gives RS-485 its noise immunity over long runs.

RS-232 also caps its slew rate at 30 V per microsecond to limit crosstalk between adjacent conductors, which is one reason its usable speed drops off with cable length. RS-485's differential scheme has no such single-ended crosstalk penalty, so it sustains far higher speeds over the same distance.

• RS-232 driver output: +5 to +15 V and -5 to -15 V; -3 V to +3 V is an undefined region.
• RS-232 logic is inverted: negative = logic 1 (mark), positive = logic 0 (space).
• RS-232 receiver: +/-3 V input sensitivity, tolerates up to +/-15 V input; slew rate limited to 30 V/us.
• RS-485 driver: differential output at least 1.5 V under load, across the A/B pair.
• RS-485 receiver: detects down to +/-200 mV differential, over a -7 V to +12 V common-mode range.
• RS-485 rejects noise because it appears on both wires and cancels at the differential receiver.

Distance, Node Count, and Termination

RS-232 is a two-endpoint standard. There is one driver and one receiver per signal direction, so there is no concept of addressing multiple devices on a line. Its practical length is about 15 m (50 ft). Later revisions of the standard replaced the fixed length with a maximum capacitive load of 2500 pF on the interconnecting cable, which in ordinary cable works out to roughly the same 15 m; using lower-capacitance cable can extend the reach somewhat. No termination resistors are used on RS-232.

RS-485 is designed as a multidrop bus. The standard defines a load budget of 32 unit loads per segment, where one unit load is a device that draws no more than 1 mA under the specified bus voltage. Because many modern transceivers present a fractional unit load (for example 1/2, 1/4, or 1/8 UL), a single segment can support more physical devices than 32 while staying within the 32-unit-load budget: at 1/8 UL, up to 256 nodes. The bus reaches roughly 1200 m (4000 ft) on one segment at lower baud rates. Longer distances or larger device counts are handled with repeaters, which start a fresh segment with its own load budget and length allowance.

Termination is required on RS-485 and is a common source of field problems. Place one 120-ohm resistor across A and B at each of the two physical ends of the bus, and only there. The 120-ohm value matches the characteristic impedance of typical twisted-pair cable so that signal energy is absorbed at the ends instead of reflecting back and corrupting data. Keep drop/stub connections short, and avoid star or ring topologies on a raw RS-485 segment; a daisy-chained bus with terminated ends is the intended layout.

• RS-232 is strictly point-to-point: one driver, one receiver, no bus addressing, no termination.
• RS-232 practical length ~15 m (50 ft), later defined as a 2500 pF maximum cable capacitance.
• RS-485 load budget: 32 unit loads per segment (1 UL = a device drawing at most 1 mA).
• Fractional-UL transceivers (1/8 UL) allow up to 256 nodes on one segment within the same budget.
• RS-485 reaches ~1200 m (4000 ft) per segment at lower baud; repeaters extend distance and node count.
• Terminate RS-485 with 120 ohms across A/B at both physical ends only; daisy-chain, keep stubs short.

The Baud-Rate vs Distance Tradeoff

Serial links do not offer full speed at full distance at the same time. As cable length grows, capacitance and signal degradation force lower data rates, so distance and baud rate trade off against each other on both standards. RS-232's original specification targeted rates up to 20 kbps within its length limit, though modern hardware routinely runs 115.2 kbps and higher over short, good-quality cable. Over its ~15 m reach, RS-232 is generally comfortable at common rates such as 9600, 19200, 38400, and 115200 baud.

RS-485 spans a much wider range. Over short spans of roughly 10 to 15 m it can run at up to 10 Mbps, while its full ~1200 m reach is typically limited to the tens of kbps. A useful mental model is that the achievable data rate falls as the run gets longer, so a designer picks the lowest baud rate that still meets the application's throughput in order to buy back distance and margin. This is why long field buses deliberately run at modest speeds rather than the maximum the transceiver supports.

Real protocols reflect this. Pelco-D and Pelco-P camera control commonly use 2400, 4800, or 9600 baud, which is more than enough for pan/tilt/zoom commands and leaves generous distance headroom. BACnet MS/TP building networks frequently run at 38400 or 76800 baud, and Modbus RTU installations often standardize on 9600 or 19200 baud so long, multidrop runs stay reliable. When a link is unreliable at the far end of a run, lowering the baud rate is one of the first and most effective adjustments.

• Distance and speed trade off: longer cable forces lower baud on both RS-232 and RS-485.
• RS-232: original standard targeted up to 20 kbps; modern hardware runs 115.2 kbps+ over short cable.
• RS-485: up to 10 Mbps over ~10-15 m, dropping to tens of kbps near the 1200 m limit.
• Pelco-D/P PTZ control commonly uses 2400, 4800, or 9600 baud.
• BACnet MS/TP commonly runs 38400 or 76800 baud; Modbus RTU often 9600 or 19200 baud.
• Lowering baud rate is a primary fix for marginal reliability at the end of a long run.

Where Each Is Used, and Half vs Full Duplex

RS-232 remains common for local, one-to-one device control. AV equipment such as projectors, flat-panel displays, matrix switchers, and audio processors frequently expose an RS-232 port so a control processor can send power, input-select, and volume commands over a short cable. Point-of-control links to a single instrument or gateway are another typical use. When the device is close and there is exactly one of it, RS-232 keeps wiring and configuration simple.

RS-485 dominates distributed, multi-device control. Access control uses it for OSDP (Open Supervised Device Protocol), the SIA-standard reader-to-controller communication that runs over RS-485 and supersedes one-way Wiegand wiring. Analog and HD-over-coax PTZ cameras carry Pelco-D and Pelco-P commands over RS-485. Building management systems use BACnet MS/TP, a token-passing protocol defined on RS-485. Industrial and metering equipment uses Modbus RTU over RS-485. In each case the draw is the same: long runs, many nodes, and noise tolerance on a single twisted pair.

RS-485 comes in two wiring schemes. Two-wire (half duplex) uses a single A/B pair shared for both directions, so only one device transmits at a time and the protocol coordinates turn-taking; this is the most common field wiring. Four-wire (full duplex) uses separate transmit and receive pairs, allowing simultaneous send and receive, often in a master-to-many topology. Either way, plan a linear daisy-chain with 120-ohm termination at both ends, a solid common/ground reference between nodes, and a baud rate chosen for the run length. Getting polarity (A vs B), termination, and grounding right at install time is what separates a stable bus from an intermittent one, and it is where a careful professional installation pays off.

• RS-232: local one-to-one AV control (projectors, displays, matrix switchers, audio processors) and single-instrument links.
• RS-485 access control: OSDP reader-to-controller communication (SIA standard), replacing one-way Wiegand.
• RS-485 camera control: Pelco-D and Pelco-P PTZ commands for analog/HD PTZ cameras.
• RS-485 building/industrial: BACnet MS/TP for BMS, Modbus RTU for metering and automation.
• Two-wire RS-485 (half duplex) shares one A/B pair; four-wire (full duplex) uses separate TX and RX pairs.
• A stable bus depends on correct A/B polarity, both-end 120-ohm termination, a common ground reference, and a run-appropriate baud rate.

Frequently asked questions