Specifications

RS-422/485 Application Note 19
© 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]
Example 2. 32 node, RS-485 network without termination
Each RS-485 node has a load impedance of 12KΩ. 32 nodes in parallel
give a total load of 375 Ω. In order to maintain at least 200 mV across 375Ω
we need a current of 0.53 mA. To generate this current from a 5V supply
requires a total resistance of 9375Ω maximum. Since 375 Ω of this total is in
the receiver load, our bias resistors must add to 9KΩ or less. Notice that very
little bias current is required in systems without termination.
Bias resistors can be placed anywhere in the network or can be split
among multiple nodes. The parallel combination of all bias resistors in a
system must be equal to or less than the calculated biasing requirements. B&B
Electronics uses 4.7KΩ bias resistors in all RS-485 products. This value is
adequate for most systems without termination. The system designer should
always calculate the biasing requirements of the network. Symptoms of under
biasing range from decreased noise immunity to complete data failure. Over
biasing has less effect on a system, the primary result is increased load on the
drivers. Systems using port powered RS-232 to RS-485 converters can be
sensitive to over biasing.
Extending the Specification
Some systems require longer distances or higher numbers of nodes than
supported by RS-422 or RS-485. Repeaters are commonly used to overcome
these barriers. An RS-485 repeater such as B&B Electronics’ 485OP can be
placed in a system to divide the load into multiple segments. Each “refreshed”
signal is capable of driving another 4000 feet of cable and an additional 31 RS-
485 loads.
Another method of increasing the number of RS-485 nodes is to use low
load type RS-485 receivers. These receivers use a higher input impedance to
reduce the load on the RS-485 drivers to increase the total number of nodes.
There are currently half and quarter load integrated circuit receivers available,
extending the total allowable number of nodes to 64 and 128.
20 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]
Chapter 3: Selecting RS-422 and RS-485 Cabling
Cable selection for RS-422 and RS-485 systems is often neglected.
Attention to a few details in the selection process can prevent the costly
prospect of re-pulling thousands of feet of cable.
Number of Conductors
The signal ground conductor is often overlooked when ordering cable. An
extra twisted pair must be specified to have enough conductors to run a signal
ground. A two-wire system then requires two twisted pair, and a four-wire
system requires three twisted pair.
Shielding
It is often hard to quantify if shielded cable is required in an application or
not. Since the added cost of shielded cable is usually minimal it is worth
installing the first time.
Cable Characteristics
When choosing a transmission line for RS-422 or RS-485, it is necessary
to examine the required distance of the cable and the data rate of the system.
The Appendix to EIA RS-422-A Standard presents an empirical curve that
relates Cable Length to Data Rate for 24 AWG twisted-pair telephone cable
that has a shunt capacitance of 16 pF/ft. and is terminated in 100 ohms (see
Figure 3.1). This curve is based on signal quality requirements of:
a). Signal rise and fall time equal to, or less than, one-half unit interval at
the applicable modulation rate.
b). The maximum voltage loss between driver and load of 6 dB.