Overview RS422 RS485 bus

About the RS422 RS485 bus can communicate with devices in an extensive area. The physical structure of the two standards is similar, but the two buses differ to the extent that the RS485 bus is networkable during the RS422 standard only point-to-point connections allowed.

Both standards transmit data via serial interfaces so that the necessary wiring is reduced to a minimum. It should be noted that the RS422 bus with a two-wire connection is only capable of simplex during the RS485 bus with only two wires already a half-duplex connection can be established. Four-wire bus connections can be established with both RS422 and RS485 full duplex.

line termination

When building a RS485 bus network as the correct line terminator must be considered. In addition, a pull-up and pull-down resistor is required to determine the signal level. Even when using RS422 should, especially at high data rates (& gt; 200kbit / s) a terminating resistor must be installed in order to reduce voltage spikes when switching.
     Scheduling can be done in different ways. One option is the omission of a termination resistor. Although this noisy environment, but also the dissipation in the network is much less because the inputs are separated. For low data rates this connection method is not feasible.
     In addition, the scheduling using a 120 Ohm termination resistor is possible. It should be noted that the RS485 bus two terminating resistors are installed while with RS422 only a terminating resistor is used. The disadvantage with this type of scheduling is the relatively high loss in the signal transmission as a part of the signal already dropped across the resistor and can not be read by the recipient.
     Can get more usable signal carried a termination with a high pass. As a result, the DC component is filtered out in the signal and suppresses only when switching distortion. The disadvantage is that the signal is not quite as smooth as when scheduling a resistance and limited by the cutoff frequency of the high pass data rate.

the specifics of each financial statements are listed again in the following table. For more information regarding the connection of the terminating resistors also refer to the section "vesting" in the article RS422 RS485 Bus


line termination
Power loss Transmission speed signal quality
Scheduling Low Low low signal rates well
high signal rates bad
Termination high high Good
High Pass average average average


Due to the differential data transmission standards both in comparison to, for example, RS232 are well protected with only one data line and ground to external electric fields, as einkoppelnde DC voltages do not affect the differential signal. Nevertheless, always shielded cables should be used to prevent a faulty transmission.


The management of the RS485 bus in the network must always be used by only one unit. The control is in this case generally a master to the traffic controls in the network and each slave responds. To enable this control addressing of individual devices is necessary that does not provide the RS422 standard. Why can not a network with RS422 devices are built.

The use of RS422 devices on a RS485 bus network is only possible if they are used as recipients have as RS485 and RS422 bus different signal levels. When installing an RS422 device in a RS485 bus network should also be noted that a RS422 device RS485 bus network so heavily loaded as 3 RS485 devices, ie the maximum number of devices per station decreases.

The following table shows again the main differences between RS422 and RS485 is.


Parameters 422 RS485
maximum number of devices 2 up to 256 transmitter / receiver
maximum data rate 10Mbit / s & gt; 10Mbit / s
common mode ± 7 V -7 to 12 V
output 2 to 10 V 1.5 to 5 V
High-impedance state 60 kOhm 12 kOhm
receiver input resistance 4 ohms 12 ohm
Sensitivity ± 200 mV ± 200 mV
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