Signal Chain Basics RS-485: Bus Current Requirements and Transceiver Drive Capability

The fact that more and more people are asking questions about the basic concepts of EIA/TIA-485, commonly known as the RS-485 data transmission standard, indicates that RS-485 will continue to play an important role in various industrial interfaces for many years to come.

In this article, we will answer many common and latest questions for you, such as:

1) How much bus current can an RS-485 transceiver drive?

2) Can it drive more than 32 unit loads?

To answer the first question, we need to examine the typical RS-485 data link shown in Figure 1. We see that in addition to driving the differential current through the termination resistors, the driver must also drive current through many receiver input impedances, as well as current through the fault protection network located on the bus. These impedances form current paths between the differential signal lines and ground, affecting the current in both the A and B signal lines to the same degree. Therefore, they can be represented as the common mode impedance, RCM.

Figure 1 Typical RS-485 data link

To define the maximum common-mode load, RS-485 uses a theoretical concept of unit load, which defines a 12kΩ common-mode load resistance. Thus, a unit load (1UL) transceiver represents an equivalent input resistance of RINEQ = 12 kΩ at each ground-relative bus end.

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 into a differential resistance of RD = 60Ω. In addition, the standard requires that this drive capability be maintained over a common-mode voltage range of VCM = –7 V to +12 V to allow for large ground potential differences between the driver and receiver grounds that are typically present in long-distance data links.

The 60 Ω differential resistance represents the resistance value of two 120 Ω termination resistors in parallel, and the total common-mode load resistance obtained by 32 unit loads is RCM = 12 kΩ / 32 = 375 Ω. The corresponding test circuit for the transceiver drive capability test under common-mode load conditions is also specified for the RS-485 standard and is shown in Figure 2.

Figure 2 Driver test circuit for common mode load

Assuming the non-inverting driver output A has a more positive bus voltage, its current is calculated as:

,

The current calculation method of the inverting output B is as follows:

.

Since the A and B outputs are constantly changing polarity during data transmission, it is best to express the output current equations using some general terms. So, the more positive output (or high output) must source the current:

,

The less positive output (or low output) must inject current:

.

Figure 3 shows the minimum output current requirement for a 5V transceiver driving a maximum common-mode load (32 UL) of RCM = 375 Ω over the specified common-mode voltage range. The parameters used to plot this graph assume VOS = 2.5 V, VOD = 1.5 V, RD = 60 Ω, and RCM = 375 Ω.

Figure 3. Bus current requirements for 5V transceivers

This graph shows that a compliant 5V transceiver must be able to source and sink up to 53 mA of output current. In practice, most RS-485 transceivers available on the market have minimum sink and source capabilities of 60 mA and above.

At this point, some important clarification regarding the maximum common-mode load of 32 unit loads is needed to dispel many common misconceptions.

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 that is shifted at the third harmonic of the system power supply frequency.

Figure 4 Unit load number as a function of GPD amplitude

summary

This article describes the minimum bus current requirement of an RS-485 standard transceiver to be approximately 60 mA and shows that the 32 UL specified common-mode load can be increased when operating at a lower common-mode voltage.

Next, Signal Chain Basics discusses how to meet the dynamic requirements of verifying the reference pin in a programmable instrumentation amplifier gain stage.

References

RS-422 and RS485 Standards Overview and System Configuration, revised May 2010, TI Application Note SLLA070D, June 2002.