RS-485 or EIA-485 is a serial interface for telecommunications like the better known RS-232. RS-485 distinguishes itself from RS-232 by a longer cable length, less susceptibility to interference and the use of a bus structure. Because of these advantages, this connection is often used as a fieldbus in industry, for example in control systems. The official name for this standard is ANSI/TIA/EIA-485-A-98.

Infrastructure RS-485 two-wire is in contrast to RS-232 half duplex, i.e. transmitting and receiving is done over the same signal lines but never at the same time. RS-485 four-wire is fully full duplex, i.e. transmitting and receiving is done over separate signal lines and can take place simultaneously.

The two-wire connection consists of 2 signal lines, an A and a B line. A is the inverted signal from B, where B is inactive high and A is inactive low. To avoid reflections from the signals, both ends must be provided with terminating resistors.

The four-wire connection consists of 4 signal lines, a TxA, TxW and RxA, RxW. There is a terminating resistor at all four ends. Outside the signal lines there is a supply line and a ground line. The standard does not specify specific connectors, usually screw connectors or Sub-D connectors are used.

Differential voltages

The use of differential voltages in combination with twisted signal lines reduces interference and allows a greater distance to be covered. EIA-485 specifies a maximum distance of 1200m and a signal voltage of -7 to 12V. However, as the distance is increased, the transfer rate will decrease. Indication: At 1.2m it is 10Mbps and at 1200m it is 100Kbps.

Network topology

RS-485 uses a bus structure; this allows up to 32 transmitters and 32 receivers to be connected to each other. However, there are RS-485 repeaters on the market that allow the maximum number of transmitters and receivers to be extended to several thousand.

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