Definition
RS485 is an industrial serial communication standard that uses differential signals, supports multi-device bus networking (one master, multiple slaves), and can transmit up to 1200 meters. It is the commonly used physical layer for protocols like Modbus RTU, DL/T 645, and industrial gateways and data collectors are often equipped with multiple RS485 interfaces.
Technical Features
RS485 uses differential transmission (A and B lines), has strong anti-interference ability, and supports 32 to 128 nodes (depending on driving capability). It is half-duplex, with master-slave polling. Common baud rates are 9600 to 115200; the farther the distance, the lower the baud rate needs to be. Termination resistors (120Ω) are used for matching and need to be added at both ends of long-distance buses. Compared with RS232, RS485 supports bus topology, longer distances, and better anti-interference, making it the mainstream serial interface for industrial sites.
Use Cases
Protocols such as Modbus RTU, DL/T 645, and M-Bus commonly use RS485 as the physical layer. PLCs, inverters, instruments, and sensors are networked via RS485. Industrial gateways and RTU with multiple RS485 ports can connect to multiple buses, collecting devices from different areas and uploading via 4G or Ethernet.
Bias and Termination on a Differential Bus
The A/B difference sets common-mode rejection
The receiver looks at the voltage difference between two wires, not at either wire relative to earth. Interference added to both wires at once leaves the difference nearly unchanged, so this interface can run farther in a workshop than a single-ended one. While transmitting, the driver pulls the two wires apart. While idle, bias resistors have to hold the bus at a defined polarity, or the receiver chatters around zero volts and the master sees a string of noise frames. That bias should usually appear only once. Bias in several places at once eats the differential amplitude.
Electrically it is a half-duplex multi-drop bus: only one driver may be active at a time. The number of nodes is limited by what the drivers can load. It is not an unlimited parallel connection. The cable should be twisted pair, with a characteristic impedance of the same order as the termination resistors at the two ends. Common practice is one matching resistor at each physical end of the bus and none on the nodes in between. As the rate rises, the same cable length starts to ring. Distance and rate have to come down together.
One bus, one master, polling
Almost every protocol on this wire uses one master that calls each slave by name, and a slave that answers only when it is called. Two masters, or one master plus a debug computer that is also polling, drive the bus at the same time, and the waveform becomes a fight between the two levels. Slave addresses must be unique on the line. It fits instruments, drives, and small controllers run together along a cable tray, with a refresh of tens of milliseconds or slower. A slave that must break in at any moment, or a mix on the same terminals with a token bus such as Profibus, conflicts in both electrical behavior and timing.
The wiring premise is a topology that is as close to a single line as possible, with short stubs. A star fork reflects at every joint. Ground the shield at one point so ground current does not circulate. Common-mode voltage has a range too. When the ground potential difference between the ends exceeds what the receiver allows, a correct differential signal can still destroy the interface. Add isolation in that case.
Termination, bias, and idle polarity
Reversed polarity is the first reason a whole bus gives no answer. A and B, D+ and D−, and some vendors' plus and minus markings do not agree. When the two ends define them differently, every slave stays silent together. Prove polarity with one known-good slave next to the master, then extend toward the far end. Second, the termination resistor is in the wrong place. Fitted at the end of a stub, or switched on at every meter, the master cannot drive the line. The near end barely works and the far end fails. Third, there is no fail-safe. Noise while idle triggers the receive interrupt, the master's frame gap is broken up, and the symptom is an occasional checksum error.
Rate and termination are often set wrong together. A short line at a low rate can work with no termination. A project copies that experience, removes the resistors, the line is extended, packets start dropping, and maintenance changes addresses instead. Restore the match at both ends before discussing the protocol. If the grounds of many devices are chained through signal ground, lightning or leakage conducts along the bus. If the isolated supply of an isolated transceiver is taken from the same non-isolated module, the isolation is not there. Finally, check rate, parity, and stop bits on every device. One device that differs will not kill the whole bus, but it will time out every time its turn arrives and stretch the whole scan.
A stub that is too long also makes the termination resistor ineffective. The resistor matches only the end of the segment it sits on. Long branches leaving a star center each reflect, while the master still shows "termination fitted." String the nodes along the trunk, and keep stubs to a very short tap. When a new instrument is added by paralleling several meters of cable from the nearest meter, the match at the trunk end no longer covers that new branch. The fault shows up only as the bus is extended.