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What Differences Exist Between RS232C, RS422 and RS485?

The standard RS422 and RS485 uses a transmission differential of 2 wires to transmit signals. Unlike the RS232C, RS422/485 signals are not referenced at 0v thus ensuring a better immunity to electrical noise. One transmits a positive signal, the other a reversed version. As a result in the presence of electrical noise, the same level of noise is superimposed on the 2 wires of the connection. Upon arrival, the receiver compares the difference of 2 signals thus eliminating the noise.
In RS422, the reception and transmission are on 2 separate tracks (or designation 4 wires) to accommodate configurations Full-duplex (simultaneous transmission and reception). In RS485, the reception and transmission takes place alternately on the same cable pair. Resistance placed on the 2 ends of the line can cancel any signal reflections caused by impedance between cable and connected devices.
In multi-point configuration, to avoid any conflict of transmission, all related equipment are held in a listening state (high impedance) when they have no information to convey. The resistance of polarization are used to define a state on the bus, thereby avoiding reception messages. The standard EIA-485 defines a half-duplex interface with the possibility of a multipoint liaison with 32 participants. The bus should include resistance to the terminal 2 ends and the cable must be twisted pair, shielded. The speed varies depending on the distances and converters.
As a result of the semi-duplex transmission, emission and reception are alternately. Hence, the interface is listening to now the permanent circuit in a state of high impedance (Tri-State).
When a slave is invited to respond, he signals the interface with RTS ON. If RTS goes high, the interface uses Tx to validate the issue and return in a state of high impedance for the first silence.
The use of RTS to drive the circuit emission is strongly recommended because in automatic mode signal Tx detection weakens every time we add a slave, since it also serves as the entry planned for the RTS. Without RTS, a network which was operating normally cannot function if we add a new station.


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Created 2008-09-11
Modified 2009-03-27
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