Main Menu Knowledgebase RS485 
RS485This page is updated November 12th 2004. It is only available in English.RS485 is a balanced multidrop communication standard. Today it has
entirely replaced the old RS422 standard. RS485 is used in a lot of
different fieldbus systems like e.g. Profibus, Interbus, Modbus and
Pnet.


Cable 
RS485 uses a shielded twisted pair cable where the shield is used as ground return, and the inner pairs are used for balanced communication. The two conductors in each pair are called A and B. RS485 is usually halfduplex, but full duplex is possible if there are two pairs in the cable. The maximum length for an RS485 cable may be calculated as: where the baud rate is specified in kBaud. If e.g. the baud rate is 3000 kBaud then the maximum length is 100 m. For baud rates less than 100 kBaud the length is close to 1200 m. The total length and the total capacitance of all drop cables (Tconnections) may be calculated as: where the baud rate is specified in KBaud. If e.g. the baud rate is 500 kBaud then the sum of the length of all drop cables shall be less than 20 m and the total capacitance of the drop cables must not exceed 0.6 nF. Note that the speed is specified in baud rate instead of bits per second. The baud rate is the reciprocal of the shortest time interval of the modulation method. For NRZ (Not Return to Zero) communication (the standard UART format) the baud rate and the bit rate is the same, but for e.g. Manchester coding the baud rate is twice as high as the bit rate. The baud rate is a good measure of the necessary bandwidth, and the relationship between the bit rate and the baud rate is a good measure of the efficiency of the modulation method. The characteristic impedance of an RS485 cable is 120 ohm according
to the standard, but cables designed for high frequency operation usually
has an impedance between 135 and 165 ohm, which limits the capacitance to
approximately 30 pF/m.


Signal Levels 
The supply voltage is 5V. The sink driver is usually able to pull the voltage on each conductor fairly close to ground (0.5V), but the source driver usually has a voltage drop of approximately 11.5V. The voltage swing on each conductor is therefore typical 33.5V, but it may be as low as approximately 2V. The receiver has an input resistor network, which makes it possible
to handle a common mode range from 7V below the negative supply voltage
(ground) to 7V above the positive supply voltage. The total common mode
range is therefore 7V to +12V. If the common mode range is exceeded the
communication may fail. It is therefore very important that the shield
(ground return) is connected in both ends. To avoid high current in
the cable shield in industrial environments it may be necessary to connect
a heavyduty parallel conductor for potential equalization between the
chassis parts!


Input Resistance 
The number of receivers on one line depends on the resistance of the
input network. A standard RS485 receiver has an input resistance of
approximately 12 kohm, which is defined as one standard load. With this
load it is possible with 32 units on one bus, but newer receivers may
have 2 or 4 times higher input resistance (1/2 or 1/4 load), which
makes it possible with up to 128 receivers.


Line Termination 
To avoid reflections the cable shall be terminated in each end with a resistor with a resistance equal to the characteristic impedance of the cable. For single master systems where the transmitter is always enabled a single resistor R1 is used between conductor A and B. For MODBUS it is a 120ohm resistor. If the transmitter is located closer to a cable end than the allowed length of a drop cable it is not necessary with a termination resistor in that end. In case of a multimaster system where the line may float and be in an undefined state if all transmitters are off, the termination network may instead consist of three resistors:
The value of the resistors R1 and R2 depends on the characteristic impedance of the cable, but for e.g. Profibus, which uses a cable with a very high impedance, R1 = 220 ohm and R2 = 390 ohm. This corresponds to a cable impedance of 170 ohm. It is very important that the two R2 resistors are fairly equal or else common mode noise will be converted to differential mode noise, which may be detected! Drop cables shall never be terminated. Note that Pnet and short MODBUS loops do not use any termination
resistors!


Cable Distances 
For a reliable communication the cable should be drawn a distance from power cables, telephone lines etc. The different cables may be divided into four categories: Category 1
Category 2
Category 3
Category 4
If cables of different categories must cross each other this shall be done in a right angle.
