B&B Electronics RS-422 Especificaciones descargar pdf (Pagina 22)

RS-422/485 Application Note 19

 Copyright B&B Electronics -- Revised October 1997

B&B Electronics -- PO Box 1040 -- Ottawa, IL 61350

PH (815) 433-5100 -- FAX (815) 434-7094

Example 2. 32 node, RS-485 network without termination

Each RS-485 node has a load impedance of 12KΩ. 32 nodes in parallel

gives 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.

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