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

(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, RCM .

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 RINEQ = 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 VOD = 1.5 V across a differential resistance of RD = 60 Ω . Furthermore, the standard requires this drive capability being maintained over a common-mode voltage range of VCM = 7 V to +12 V to allow for large ground potential differences between driver and receiver grounds, which are commonly encountered in long distance data links.

While the 60 Ω differential resistance represents the resistor value of two 120 Ω termination resistors in parallel, the 32 unit loads yield a total common-mode load resistance of RCM = 12 k Ω / 32 = 375 Ω . The corresponding test circuit for testing a transceiver’s drive capability under common-mode loading is also specified in RS-485 and shown in Figure 2 .

 

Figure 2. Driver test circuit for common-mode loading.

Assuming the non-inverting driver output A provides the more positive bus voltage, its current is given with:


while the current of the inverting output B is determined through:

Because during the transmission of data the A and B outputs change polarity continuously, it is better to express the output current equations in generic terms. Thus, the more positive output (or high output) must source a current of:

while the less positive output (or low output) must sink a current of:

Figure 3 shows the minimum output current requirements for a 5 V transceiver driving the maximum common-mode load of RCM = 375 Ω, or 32 ULs, over the specified common-mode voltage range. The parameters used to generate this diagram were assumed with:

VOS = 2.5 V, VOD = 1.5 V, RD = 60 Ω, and RCM = 375 Ω.

 

Figure 3. Bus current requirements for a 5 V transceiver.

The diagram shows that a standard compliant 5V transceiver must be able to source and sink output currents of up to 53 mA. In practice most RS-485 transceivers available on the market possess minimum sink and source capabilities of 60 mA and more.

At this point some important clarifications with regards to the maximum common-mode loading of 32 unit loads might be appropriate to remove common misperceptions.

  • The maximum common-mode loading of 32 unit loads specified in RS-485 accounts for any common-mode load that exists between the differential signal pair and signal ground, not just receiver inputs. For example, an external failsafe resistor network already utilizes total of 22 ULs alone, thus leaving only 10 ULs available for receiver inputs. These remaining 10 ULs can be utilized either by using 10 x 1 UL transceivers, or at best, 80 x 1/8 UL transceivers.
  • The maximum loading of 32 ULs is specified for entire VCM range from –7 V to +12V. As can be seen in Figure 3, narrowing the VCM range reduces the output currents and leaves the driver with some current reserves. These reserves can then be utilized to drive more unit loads. In data links with small ground potential differences (GPD) between driver and receiver grounds this principle can be applied, which answers our second question posed at the beginning.


Figure 4 shows the number of unit loads as a function of the GPD amplitude. Note that the GPD is not a DC voltage but an AC voltage alternating at the third harmonic of the system supply’s mains frequency.

 

Figure 4. Number of unit loads as a function of GPD amplitude.

Summary

This article describes that the minimum bus current requirement for an RS-495-compliant transceiver is about 60 mA, and shows that the specified common-mode loading of 32 ULs can be increased when operating at lower common-mode voltages.

Please join us next month, when the Signal Chain Basics will cover how to examine satisfying the dynamic requirements of the reference pin in programmable- and instrumentation-amplifier gain stages.

References


About the Author


Thomas Kugelstadt is a Senior Applications Engineer at Texas Instruments where he is responsible for defining new, high-performance analog products and developing complete system solutions that detect and condition low-level analog signals in industrial systems. During his 21 years with TI, he has been assigned to various international application positions in Europe, Asia and the U.S. Thomas is a Graduate Engineer from the Frankfurt University of Applied Science.

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