RS-485 termination and biasing

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TIA/EIA-422-B and TIA-485-A Termination
Termination is used to match the impedance of a transmitting or receiving node to the impedance of the transmission line used.

If the impedances are mismatched the transmitted signal cannot be fully absorbed by the load and some portion of the signal will be reflected back onto the transmission line. This reflected signal will travel up and down the cable reducing in amplitude over time.

The disadvantages of terminating are:
• Driver loads are increased.
• Biasing requirements are changed.

Whether termination is required, on a network, should be based upon the total cable length and the data rate employed. If all signal reflections will be damped out prior to the centre of a data bit, at which point the receiver will be sampling, termination will not be required.

For example the propagation delay of any cable can be calculated from its length and propagation velocity (typically 66-75% of the speed of light (c)).

If a cable of 100m has a round trip of 200m and a propagation velocity of 66% of c, one round trip is completed in approximately 1μs.
Assuming that the reflections are completely damped after 5 round trips, the signal will stabilize after 5μs.

At 9600 baud each bit is 104μs wide. As the signal is stable well before the center of the bit termination should not be required.

At 115.2k baud each bit is 8.7μs wide. As the signal is not stable before the center of the bit termination will be required.

The above calculation shows that, we should be able to have a network length of about 160m before termination is needed at 115.2k baud.

Termination resistors should only be placed at the extreme ends of a network, and no more than two termination resistors should be used per network.

For a 2-wire network the termination resistors are typically fitted to the slave nodes at the extreme ends of a network.

For 4-wire networks the termination resistors are typically fitted to the receive pair on the slave nodes at the extreme ends of a network.

TIA/EIA-422-B and TIA-485-A Biasing
When a TIA-485-A network is idle all nodes are set to receive data and therefore all drivers are tri-stated. Without anything driving the network the state of the line is unknown.

This is not strictly applicable to a true TIA/EIA-422-B network as this should always have the master node transmitter enabled.

However, due to the common use of cost effective RS485 drivers and automatically detecting when the driver requires enabling, it is recommended to add biasing to these networks too.

If the voltage at the receiver inputs is less than ±200mV the receiver output logic level will show that of the last bit received.

In order to maintain the correct idle state bias resistors can be added to the transmission lines. A pull-up resistor, typically to +5V, is added to RX+ and a pull-down, to ground, is added to RX-.

The bias resistor values are determined by the network load, including terminations if fitted:

When termination resistors are fitted the loading effect of these is greater than the nodes, which have a typical load of 12kΩ per node. This means that the bias resistor values are approximately 685Ω regardless of the number of nodes.

When termination is not fitted the bias resistors can vary from 122kΩ for two nodes to 4.5kΩ for 32 nodes.

Bias resistors can be added at any point on the network or can be split among multiple nodes. The parallel combination of all bias resistors on a network should be equal to or less than the biasing requirements.

They are typically added to the master node.

TIA/EIA-422-B and TIA-485-A Network Topology
Both TIA/EIA-422-B and TIA-485-A networks should be daisy-chained, for reliable operation.

Best practice for 2-wire TIA-485-A networks is to have the master node in middle of the network with bias resistors fitted and the two end slave nodes to have terminations fitted.

TIA/EIA-422-B and TIA-485-A Compatibility
To ensure compatibility between the system and the application slave devices it is necessary to ensure that the format, speed and duplex settings match.

TIA/EIA-422-B and TIA-485-A devices will often use the labels A and B for their connections.
It should be noted that these pin labels are not used consistently between manufacturers, are not always used as shown in the specifications and can even be inconsistent between different devices from the same manufacturer.

For example:
The Linear Tech LTC1535 labels the Non-inverting input as A whilst the Linear Tech LTC1387 labels the inverting input as A.
The TIA/EIA-422-B specification labels the inverting input as A with an alternate label of ‘-‘, and the non-inverting input as B with an alternate label of ‘+’.
For correct operation care must be taken when connecting the devices onto the network.