SIGNAL CHAIN BASICS #52: RS-485—Bus current requirements and transceiver drive capability

April 19, 2011 //By Thomas Kugelstadt
SIGNAL CHAIN BASICS #52: RS-485—Bus current requirements and transceiver drive capability
Thomas Kugelstadt, Senior Applications Engineer, Texas Instruments provide answers to some common questions such as how much bus current must an RS-485 transceiver be able to drive? And, is it possible to drive more than 32 unit loads?

(Editor's note : Signal Chain Basics is an ongoing (and popular) series; you can click here for a complete, linked list of all previous installments of the series.)

The proof that RS-485 will continue to be the workhorse amongst industrial interfaces for many years to come is presented by the increasing number of questions asked regarding the basic concepts of EIA/TIA-485, the data transmission standard commonly known as RS-485.

In this article we provide answer to the more common and most recent concerns such as:

1)     How much bus current must an RS-485 transceiver be able to drive?

2)     And, is it possible to drive more than 32 unit loads? 

To answer the first question we look at a typical RS-485 data link shown in Figure 1.


 Figure 1. Typical RS-485 data link.

Observe that, in addition to driving differential current through the termination resistors, a driver must also drive current through a number of receiver input impedances and fail safe networks present on the bus. Because these impedances present current paths between the differential signal lines and ground, they affect the current flow in both the A and B signal lines equally. Hence, they are designated as common-mode impedances, R CM.

In order to define maximum common-mode loading, RS-485 has introduced the theoretical concept of a unit load, which defines a common-mode load resistance of 12 k Ω. Thus, a one unit load (1UL) transceiver represents an equivalent input resistance of R INEQ = 12 k Ω at each bus terminal with respect to ground .

RS-485 specifies that a transceiver must be able to drive a total common-mode load of up to 32 unit loads, while providing a differential output voltage of V OD = 1.5 V across a differential resistance of R D = 60 Ω. Furthermore, the standard requires this drive capability being maintained over a common-mode voltage range

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