Serial Communication: RS-232, RS-422 and RS-485

Why serial still matters

Serial links are old, slow and everywhere. A meter, a drive, a barcode reader, a temperature controller or a PLC programming port very often speaks RS-232 or RS-485, and a USB adapter plus a free program is often the quickest way to find out whether a device is alive. Serial is also the easiest network to understand completely: one pair of wires, one character at a time. If you can troubleshoot serial, the habits (match the settings, check polarity, terminate properly, work from the cable up) carry to every other network.

How a serial character is built

A serial line sends one bit at a time at a speed called the baud rate. There is no clock wire, so each character is wrapped in markers the receiver uses to find the bits: the line idles at logic 1; a start bit (0) announces a character; then the data bits follow, least significant bit first; an optional parity bit allows a simple error check; and one or two stop bits (1) return the line to idle. Both ends must agree on all of these.

Setting	Meaning	Common values
Baud rate	Bits per second	9 600, 19 200, 38 400, 115 200
Data bits	Bits of data per character	8 (Modbus RTU), 7 (ASCII protocols, some PLC links)
Parity	None, even or odd: an extra bit that makes the count of 1 bits even or odd; catches a single flipped bit	Even (Modbus default), none
Stop bits	Idle time after each character	1; 2 when parity is none (Modbus)
Flow control	Hardware (RTS/CTS) or software (XON/XOFF) handshake	None for most industrial links

Character times

Baud rate	One bit	8N1 character	8E1 character	Characters per second (8N1 / 8E1)	Modbus RTU gap (3.5 characters)
1 200	0.833 ms	8.333 ms	9.167 ms	120 / 109	32.08 ms
2 400	0.417 ms	4.167 ms	4.583 ms	240 / 218	16.04 ms
4 800	0.208 ms	2.083 ms	2.292 ms	480 / 436	8.02 ms
9 600	0.104 ms	1.042 ms	1.146 ms	960 / 873	4.01 ms
19 200	0.052 ms	0.521 ms	0.573 ms	1 920 / 1 745	2.01 ms
38 400	0.026 ms	0.260 ms	0.286 ms	3 840 / 3 491	1.75 ms
57 600	0.017 ms	0.174 ms	0.191 ms	5 760 / 5 236	1.75 ms
115 200	0.009 ms	0.087 ms	0.095 ms	11 520 / 10 473	1.75 ms

RS-232, RS-422 and RS-485 compared

These are electrical standards: they say what voltages represent bits and how many devices may share the wires. They do not define the message format; Modbus, DF1, Host Link and the rest sit on top.

	RS-232	RS-422	RS-485
Signaling	Single-ended, about ±3 to ±15 V against a common ground	Differential, one pair for each direction	Differential, one shared pair (2-wire) or two pairs (4-wire)
Receiver threshold	±3 V	±200 mV difference	±200 mV difference
Devices	2 (point to point)	1 driver, up to 10 receivers	Up to 32 unit loads (256 with 1/8-load transceivers)
Wires	TXD, RXD, ground (plus handshake lines)	4 wires and a reference, full duplex	2 wires half duplex, or 4 wires
Maximum length	About 15 m at 9 600 baud	About 1 200 m	About 1 200 m
Maximum speed	115 kbit/s typical (up to 1 Mbit/s short)	10 Mbit/s at 12 m	10 Mbit/s at 12 m
Noise immunity	Poor	Good	Good
Typical use	Programming ports, barcode readers, instruments, PC to PLC	Point-to-multipoint, older drives and PLC links	Modbus RTU, PROFIBUS DP, meters, drives, sensors

RS-232 in practice

DB9 pin	Signal (DTE, a PC)	Direction	Used for
1	DCD, carrier detect	in	Modems; usually unused
2	RXD, receive data	in	Data from the other device
3	TXD, transmit data	out	Data to the other device
4	DTR, data terminal ready	out	Handshake; sometimes used to power or reset
5	GND, signal ground		The reference for all the signals
6	DSR, data set ready	in	Handshake
7	RTS, request to send	out	Handshake; also RS-485 direction control on some adapters
8	CTS, clear to send	in	Handshake
9	RI, ring indicator	in	Modems; sometimes carries +5 V on special ports

• A PC is a DTE (data terminal), a modem is a DCE. A DTE talks to a DCE with a straight cable (pin 2 to 2, 3 to 3). Two DTEs need the transmit and receive lines crossed: a null-modem cable. Many PLC programming ports are wired as DTEs, which is why a PC needs a crossed cable to some and a straight cable to others. If nothing arrives, try the other type of cable first.
• Only three wires matter for most links: RXD, TXD and ground. Handshake wires are used only when the device manual asks for them.
• RS-232 does not need terminators or bias. It does need a common ground, and it is limited by cable capacitance: about 15 m at 9 600 baud, less at higher speeds.
• Not every DB9 is RS-232. Some PLC ports are RS-485 or RS-422 on a DB9 and carry 5 V or 24 V on spare pins. Read the manual before you connect a PC adapter, or you may damage one of the two ports.
• Connector gender. The PC end is usually a male DB9 (pins) and the device end female, but not always. A gender changer fixes the fit and changes nothing electrically.
• Long RS-232 runs. Past 15 m, convert to RS-485 (an RS-232 to RS-485 converter at each end), use a lower baud rate, or move to Ethernet with a serial device server.

RS-485 in practice

RS-485 carries a bit as the voltage difference between two wires, usually called A and B (or D− and D+). Noise that hits both wires equally cancels out, which is why RS-485 works over long cables next to motors. A line driver turns the signal on and a receiver reads it; on a shared pair only one driver may be on at a time, so each device switches its driver off when it is not sending (direction control).

Mode	Wires	How it works	Typical use
2-wire half duplex	A, B (+ reference)	One pair; the master sends, then listens	Modbus RTU, PROFIBUS, most drives and meters
4-wire full duplex	TX pair, RX pair	The master has a transmit pair to all slaves and a receive pair from them; slaves still take turns replying	Older equipment, some serial PLC networks (RS-422 style)
Point to point	A, B	Two devices only; still terminate	Drive to PLC, instrument to PC

• Direction control. A converter or port must enable its driver only while sending. Good USB-RS485 adapters do this automatically; some PLC ports use the RTS line or a configuration setting. A driver left on blocks every other device on the bus.
• Turnaround. After sending, the master must switch to receive quickly enough to catch the reply; a slow adapter or a software delay causes lost responses at high baud rates.
• A and B are not standard. Some makers call the same wire A and others B, and +/− is used as well. If nothing works, swapping the pair on one device is the first test.

Wiring an RS-485 bus

• Daisy-chain, never a star. Run the cable from device to device. Any branch (stub) should be as short as possible: under about 1 m at 9 600 baud, shorter at higher speeds.
• Twisted, shielded pair with 120 Ω characteristic impedance (for example Belden 9841). The twist is what makes the noise cancel; do not use untwisted bell wire or two wires from a multicore cable.
• Terminate both ends, and only the ends, with a resistor equal to the cable impedance (120 Ω) across A and B. Many devices have a switchable terminator; check that exactly two are on.
• Bias once (next section), at one point, so the idle line has a defined level.
• Common reference. Connect the signal grounds of the devices through the third conductor (or the shield through a resistor) so that every receiver sees the signals within its common-mode range (−7 V to +12 V).
• Shield at one end, or through a bond. See the grounding section below and Cabling, Grounding and Noise.
• Keep it away from power cables and VFD motor leads.
• Repeaters extend a bus past the node or distance limit and isolate segments; each segment has its own terminators.

Termination and bias

Two resistors do two different jobs. Termination (120 Ω across the pair at each end) matches the cable’s impedance so that fast edges are absorbed and do not reflect back along the cable and corrupt later bits. Bias (a pull-up resistor from B to +5 V and a pull-down from A to ground) makes the idle line sit at a clear logic level, so receivers do not read noise as data when nobody is transmitting. Termination is needed on both ends; bias is needed once on the whole bus (usually at the master or in the converter).

Measurement (power off, bus disconnected from the master)	Reading	Meaning
Resistance across A and B	About 60 Ω	Two 120 Ω terminators: correct
Resistance across A and B	About 120 Ω	Only one terminator
Resistance across A and B	Very high (open)	No terminator
Resistance across A and B	Below about 40 Ω	More than two terminators, or a short
Voltage A to B (power on, idle)	About +0.2 V to +5 V with B positive	Biased and idle; a reading near 0 V means no bias

Distance, speed and the number of devices

Cable length falls as speed rises, because the cable’s capacitance rounds the edges until the bits blur together. At 9 600 baud the practical limit is the 1 200 m of the standard; at 1 Mbit/s it is around 120 m; at 10 Mbit/s a few tens of meters. Most Modbus RTU systems run at 9 600 or 19 200 baud, which leaves plenty of margin.

Unit loads

The “32 devices” limit counts unit loads, a measure of how much each receiver loads the line. A classic transceiver is 1 unit load, so 32 fit on a bus. Modern transceivers are rated 1/2, 1/4 or 1/8 unit load, allowing 64, 128 or 256 devices; but the cable resistance and capacitance still limit the practical number. Add up the unit loads from the datasheets.

Grounding, isolation and surge protection

Two panels often have a different ground potential: a few volts is common, and tens of volts occurs near large drives and welders. A cable that joins their signal grounds carries the difference as a current, adding noise, and a difference above about 7 V puts the signals outside the receivers’ common-mode range, which can destroy the ports.

Measure	What it does	When to use it
Isolated transceivers or converters	Separate the signal ground of each device with a barrier (typically 1 to 3 kV)	Whenever two panels, or a laptop and a panel, are joined: the safe default
Common reference through 100 Ω	Joins the signal grounds through a resistor that limits ground current	Non-isolated ports on the same bonded system
Shield grounded at one end	Stops hum without a ground loop	Low-frequency noise; short runs
Shield bonded at both ends	Blocks radio-frequency noise; needs equipotential bonding so that ground current does not use the shield	Heavy drives, long cables, with a bonding conductor beside the cable
Surge protectors (TVS or gas tubes)	Clamp lightning and switching spikes on long outdoor runs	Cables between buildings; use fiber where possible
Fiber optic link	No electrical connection at all	Between buildings and across large ground differences

Settings that must match

Setting	Why it matters	If wrong
Baud rate	The receiver samples at its own clock; the speeds must agree to within a few percent	Garbage characters, framing errors
Data bits	7 or 8 data bits change where the stop bit falls	Garbage or shifted data
Parity	An even-parity receiver rejects characters from an odd or no-parity sender	Parity errors; frames ignored
Stop bits	1 or 2; usually tolerated if the sender uses more than the receiver expects	Occasional framing errors
Protocol	RTU against ASCII; Modbus against DF1 or Host Link	No reply
Station address	Unique on the bus; valid range for the protocol	No reply, or two devices answering at once
Timeout and retries	How long the master waits and how often it tries again	False alarms, or a slow recovery

Write the settings of every device on a table beside the drawings. A good habit when a link does not work is to put the device back to its factory defaults and use those.

Converters and adapters

Device	What it does	Buying notes
USB to RS-232 adapter	Gives a laptop a COM port (DB9)	FTDI or Silicon Labs chips; avoid counterfeit Prolific chips; isolated versions for plant use
USB to RS-485 / RS-422 adapter	Screw terminals for A, B and ground	Automatic direction control; isolation; a terminator switch
RS-232 to RS-485 converter	Extends or joins networks	Isolation, DIN-rail power, bias and terminator options
Serial device server	Puts a serial device on Ethernet (a COM port over TCP)	Raw TCP, Modbus gateway and virtual COM modes; power and surge protection
Repeater / isolator	Extends a bus or splits ground	Each side has its own terminators
Protocol gateway	Translates, for example Modbus RTU to Modbus TCP or EtherNet/IP	Check the register mapping and the poll model

The Sniffers, Analyzers and Serial Tools page explains how to use adapters to test and sniff a serial link.

Common mistakes

Mistake	What you see	Fix
Straight cable where a null modem is needed (or the reverse)	No data at all	Swap pins 2 and 3, or fit a null-modem adapter
A and B swapped on one device	No reply, or the line seems dead	Swap the pair on one device
Wrong baud rate or parity	Garbage characters, framing errors	Match the settings at both ends; scan the baud rates
Terminators missing or in the middle	Errors rising with baud rate or cable length	Exactly two terminators, at the two ends
Star wiring or long stubs	Random errors, some devices fail	Daisy-chain; shorten stubs
Two devices with the same address	Garbled replies or none	Give each its own address
Bias in several places	Drivers overloaded, weak signals	Bias at one point only
No common reference	Works one day, fails the next	Connect the reference or use isolated ports
Different ground potentials with no isolation	Intermittent faults, burnt ports	Isolated converters; bond the panels
USB adapter not set to the right COM port	Terminal shows nothing	Check Device Manager and the software setting

Related

• Industrial Communication: the overview of this guide.
• Modbus RTU, ASCII and TCP: the protocol that most often runs on RS-485.
• Cabling, Grounding and Noise: cable types, shielding and routing.
• Sniffers, Analyzers and Serial Tools: adapters, loopback tests and serial sniffers.
• Troubleshooting Industrial Networks: a step-by-step method for serial faults.
• Modbus Frame and CRC Calculator.

FAQ

What baud rate should I use?

Start with 9 600 or 19 200. Higher speeds work on short, clean buses, but long cables, many devices and noise favor lower rates. Every device on the bus must use the same setting.

Do I always need a terminator on RS-485?

On a long cable or at high speed, yes, at both ends. A short bench setup at 9 600 baud may work without, but adding the terminators is good practice and costs nothing.

Can I run RS-485 in a star?

Not reliably. Stubs cause reflections. Use a daisy chain, or put a repeater or hub in each branch so that each branch is its own bus.

Why does my RS-232 cable stop working over 20 meters?

The cable’s capacitance rounds the edges, and RS-232’s single-ended signal picks up noise. Convert to RS-485 or use a lower baud rate and a low-capacitance cable.

What is the difference between RS-422 and RS-485?

RS-422 has one driver and up to ten receivers on separate pairs. RS-485 allows many drivers on a shared pair, and its drivers can be switched off. A device with RS-422 can often be connected to an RS-485 bus in 4-wire mode.

Why do some adapters need an RTS signal?

To switch the driver between transmit and receive. Good adapters do it automatically; with a simple one, the software must raise RTS while sending.

References

• TIA/EIA-232-F, Interface between Data Terminal Equipment and Data Circuit-Terminating Equipment.
• TIA/EIA-485-A, Electrical Characteristics of Generators and Receivers for Use in Balanced Digital Multipoint Systems; TIA/EIA-422-B.
• Texas Instruments, The RS-485 Design Guide (application report SLLA272) and RS-485 Basics: Termination and Bias.
• Modbus Organization, MODBUS over Serial Line Specification and Implementation Guide V1.02 (settings, timing, wiring).
• Belden, Industrial Cable Technical Data: RS-485 and PROFIBUS cables.

Last reviewed: October 2026. Speeds, lengths, node counts and menu names are typical: confirm against the specification and the manufacturer’s documentation for the devices you use.