RS-422/485 Termination and Biasing

Termination and Biasing are concepts that only apply to differential wiring. As such, they only apply to RS-422 and RS-485 and not RS-232. The Termination and Biasing concepts are described in detail below. First, however, we will describe the options provided by the RMC SERIAL module.

For RMC SERIAL modules with hardware revision 2 or later, biasing and termination can be independently selected for both wire pairs. For RMC SERIAL hardware revision 1, biasing and termination must be enabled and disabled together and are only available on the Tx/Rx wire pair and not on the Tx pair.

To change the termination and biasing options for the RMC SERIAL module:

1. On the Tools menu, click Module Configuration.

2. In the Slots list, click the Serial line.

3. Click Slot Options.

4. In the Serial Module Options dialog box, ensure that RS-422/RS-485 (4-wire) or RS-485 (2-wire) is selected.

5. Click Advanced.

6. Check or un-check the Enable Biasing Circuit and Enable Termination check boxes for the transmitter and receiver (for 4-wire) or transceiver (for 2-wire). The diagram in the dialog box will update accordingly.

Note: As described above, hardware revision 1 does not allow biasing and termination to be independently selected, and only allows enabling them on the Rx/Tx pair.

7. Click OK.

8. Click Update RMC.

This step will save the settings to Flash memory.

The Update Module Configuration dialog box will be displayed to indicate the progress. If the module could not be reset automatically, you may be prompted to reset the module manually.

9. In the RMC Configuration dialog box, click Close.

The values of the resisters and capacitors in version 2 and later of the RMC SERIAL's termination/biasing circuit are shown in the diagram below:

As described above, hardware revision 1 differs in its biasing and termination. It has the following differences:

• There is no biasing or termination on the Tx wire pair.

• There is no capacitor in the Rx/Tx termination circuit.

• The values of the resisters in the Rx/Tx biasing circuit are 2.2 kW instead of 1.15 kW.

Termination

Cable termination is a way of absorbing transmitted energy at the end of a network. This prevents signal reflections from bouncing back towards the transmitter and potentially upsetting signal quality and communications.

The termination resistor should match the characteristic impedance of the cable being terminated. The effective impedance of the RMC SERIAL's termination resistor and biasing resistors is 114W. Therefore, cabling with impedance of 100W to 120W is recommended.

Termination should be placed at the end of the network for each wire pair. The diagrams in Serial Network Topologies show the correct locations of the termination:

• For RS-422 (4-wire, point-to-point), terminate the receiver of each wire pair.

• For RS-485 (2-wire, point-to-point or multi-drop), terminate the wire pair at each end of the network.

• For RS-485 (4-wire, point-to-point or multi-drop), terminate the receivers of each end device in the chain, and the transmitter of the last slave (but not master).

Termination is not required on all differential networks, but it does typically extend the maximum cable length. The following chart shows the maximum cable lengths at various baud rates with and without termination:

Note: The maximum cable length is the length of the entire network and not just the distance between nodes on the network.

Termination vs. Cable Length:

Cable Length Derivation

The values presented in the chart above are based on 24AWG cable with capacitance of 16 pF/ft and the following reasoning. Signals travel through a cable at approximately 66% of c or 0.66 ft/ns. It is assumed that a signal transition will dampen out after three round trips in the cable. This damping must occur before the bit is sampled or within half a bit time. One bit time is equal to the reciprocal of the baud rate.

Example:

Compute the cable length for 115,200 baud RS422.

First, we compute a half bit time at this baud rate.

Next, we convert this time to the distance the signal would travel in this time, assuming a speed of 0.66 ft/ns as described above:

Since it requires three round trips for the signal transition to dampen and each round trip is twice the length of the cable, the total distance in feet is divided by six to get the final unterminated cable length:

This value is then rounded down to allow for inexact cable velocities and damping rates, giving us 475 ft.

Biasing

RS-422 and RS-485 indicate a binary 1 when the A line is at least 200 mV negative with respect to B, and a binary 0 when A is at least 200 mV positive with respect to B. It is important that the lines always be in a known state, not only when being driven. Biasing forces the network into a known state when the lines are idle and therefore otherwise not driven.

Hardware revisions 2 and later of the RMC SERIAL contain a 68 pF capacitor in series with the 120W termination resistor. This keeps the resister from loading the network when the network is idle, and maintains a known state without biasing. However, the software selectable biasing circuit can be used to bias the line when resistive termination is present on the other end of the network.

Hardware revision 1 of the RMC SERIAL module does not have the capacitor in its termination circuit and therefore will require biasing.

Biasing forces a valid state onto the network by allowing current to flow across the termination resistor.

Example:

This example assumes that there is a single master and two RMCs on the network. Compute the voltage across a 120W termination resistor when using 1150W biasing resistors.

First, we calculate how much DC resistance will be between the biasing resistors. Calculating the parallel resistance of all DC terminations and node input impedances does this. For a single master and two RMCs we have the following components:

• Master load: 1 unit load, which is defined as 12 kW.

• RMC loads: ¼ unit load each, which is 48 kW.

• Termination Resister in the RMC: 120W

Therefore, putting all of the resistances in parallel yields the following:

Then, we calculate how much DC resistance the network has between power rails:

Next, we calculate how much current is flowing through this DC resistance:

Finally, we calculate the voltage drop across the termination resistor:

This value is greater than the ±200mV difference required by the TIA/EIA standards and constitutes a valid binary 0 state.

Copyright (c) 1997-2015 by Delta Computer Systems, Inc.