RS-485

This page is updated November 12th 2004. It is only available in English.

RS-485 is a balanced multidrop communication standard. Today it has entirely replaced the old RS-422 standard. RS-485 is used in a lot of different fieldbus systems like e.g. Profibus, Interbus, Modbus and P-net.

Cable

RS-485 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. RS-485 is usually half-duplex, but full duplex is possible if there are two pairs in the cable.

The maximum length for an RS-485 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 (T-connections) 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 RS-485 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 1-1.5V. The voltage swing on each conductor is therefore typical 3-3.5V, 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 heavy-duty 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 RS-485 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 120-ohm 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:

• One resistor R2 from ground (0V) to conductors A.
• One resistor R1 between the two conductors A and B.
• One resistor R2 from conductor B to the positive supply voltage (5V).

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 P-net 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

• RS-485 cables
• Fieldbus and LAN cables (Profibus, CAN, ASi, Ethernet etc.)
• Shielded cables for digital data and data communication (RS-232 etc.)
• Shielded cables for low voltage (<25V) digital and analog signals
• coaxial cables (video, audio, TV etc.)
• Cables for DC voltages below 60V

Category 2

• Cables for DC voltages between 60V and 400V
• Cables for AC voltages between 25V and 400V

Category 3

• Cables for DC and AC voltages above 400V
• Telephone cables

Category 4

• Cables of category 1 and 2 where high voltage or lightning transients may occur.

cm	Cat. 1	Cat. 2	Cat. 3	Cat. 4
Cat. 1	0	10	20	50
Cat. 2	10	0	10	50
Cat. 3	20	10	0	50
Cat. 4	50	50	50	0

inches	Cat. 1	Cat. 2	Cat. 3	Cat. 4
Cat. 1	0	4	8	20
Cat. 2	4	0	4	20
Cat. 3	8	4	0	20
Cat. 4	20	20	20	0

If cables of different categories must cross each other this shall be done in a right angle.