Specifications

RS-422/485 Application Note 17
© Copyright B&B Electronics -- Revised 2506
B&B Electronics Mfg Co – 707 Dayton Rd - PO Box 1040 - Ottawa IL 61350 - Ph 815-433-5100 - Fax [phone]
B&B Electronics Ltd – Westlink Comm. Pk – Oranmore, Galway, Ireland – Ph 353-91-792444 – Fax [phone]
System designers interested in AC termination are encouraged to read National
Semiconductors Application Note 903
2
for further information. Figure 2.3
illustrates both parallel and AC termination on an RS-485 two-wire node. In
four-wire systems, the termination is placed across the receiver of the node.
Figure 2.3 Parallel and AC Termination
Biasing an RS-485 Network
When an RS-485 network is in an idle state, all nodes are in listen
(receive) mode. Under this condition there are no active drivers on the
network. All drivers are tristated. Without anything driving the network, the
state of the line is unknown. If the voltage level at the receiver’s A and B
inputs is less than ±200 mV the logic level at the output of the receivers will be
the value of the last bit received. In order to maintain the proper idle voltage
state, bias resistors must be applied to force the data lines to the idle condition.
Bias resistors are nothing more than a pullup resistor on the data B line
(typically to 5 volts) and a pulldown (to ground) on the data A line. Figure 2.4
illustrates the placement of bias resistors on a transceiver in a two-wire
configuration. Note that in an RS-485 four-wire configuration, the bias
resistors should be placed on the receiver lines. The value of the bias resistors
is dependent on termination and number of nodes in the system. The goal is to
generate enough DC bias current in the network to maintain a minimum of 200
mV between the B and A data line. Consider the following two examples of
bias resistor calculation.
2
Refer to Chapter 7 for information on National Semiconductors Application
Notes.
18 RS-422/485 Application Note
© Copyright B&B Electronics -- Revised 2506
B&B Electronics Mfg Co – 707 Dayton Rd - PO Box 1040 - Ottawa IL 61350 - Ph 815-433-5100 - Fax [phone]
B&B Electronics Ltd – Westlink Comm. Pk – Oranmore, Galway, Ireland – Ph 353-91-792444 – Fax [phone]
Figure 2.4 Transceiver with Bias Resistors
Example 1. 10 node, RS-485 network with two 120 Ω termination
resistors
Each RS-485 node has a load impedance of 12KΩ. 10 nodes in parallel
give a load of 1200 Ω. Additionally, the two 120 Ω termination resistors result
in another 60 Ω load, for a total load of 57 Ω. Clearly the termination resistors
are responsible for a majority of the loading. In order to maintain at least
200mV between the B and A line, we need a bias current of 3.5 mA to flow
through the load. To create this bias from a 5V supply a total series resistance
of 1428 Ω or less is required. Subtract the 57 Ω that is already a part of the
load, and we are left with 1371 Ω. Placing half of this value as a pullup to 5V
and half as a pulldown to ground gives a maximum bias resistor value of 685Ω
for each of the two biasing resistors.
Bias Resistor
Bias Resistor