RS485 is the only differential interface allowing for robust data transmission over miles — long distances at reasonably high data rates of up to 100kbps. Preferably bus nodes are connected by daisy-chain, or a single cable run that connects one node to the next. Because each node receives its power supply from a different point in the electrical installation, ground potential differences (GPDs) between bus nodes are common.
These GPDs increase the common-mode voltage on the bus and can cause data errors or device damage, if not properly dealt with. RS485-compliant transceivers tolerate GPDs of up to ±7V. Higher GPDs however require special transceivers with a higher input voltage range, or galvanically isolated transceivers.
Some engineers use ground wires between bus nodes in order to tie the transceiver grounds to a defined voltage potential. This method can have dangerous outcomes as it shortens multiple ground potential differences together causing prohibitively large ground currents that literally burn ground wires.
On the basis of a single point-to-point interface, this article shows that differential transceivers are designed to operate without ground wires.
The differential driver has a H-bridge output stage that drives current from terminal A to terminal B and vice versa, depending on the logic states at data input, D (Figure 2).
Because the line voltages, VA and VB , switch between two positive potentials, the driver can be modeled as a signal source with a common-mode output offset, VOS , and a superimposed differential voltage of ±VOD /2 (Figure 3).
The differential receiver’s input stage consists of a resistor network that attenuates large input signals and biases the comparator inputs with respect to receiver ground (Figure 4). By referencing the inputs to receiver ground, the receiver can build the voltage difference between VA and VB without a ground connection between driver and receiver ground.
Voltage divider action between the input resistors (Rin) and the bias resistors (Rb) attenuates the input signal (common-mode and differential voltage components likewise) by a factor of ten. This prevents the comparator inputs from saturation (Figure 5).
Voltage divider action between the bias resistors biases the comparator inputs with VCC /2 potential, thus enabling the receiver to work from a single-supply voltage.
The data link in Figure 6 applies the previously gained information by showing the GPD as a common-mode voltage that adds to the driver output offset. In order for the data link to work reliably, the sum of the total common-mode voltage and the maximum signal must not exceed the specified maximum receiver input voltage:
VIN-max ≥ VCM + VD /2 (1)
In an RS485 data link the driver offset typically is half the driver supply, e.g., 2.5V. Adding a GPD of ±7V yields a common-mode voltage range from 9.5V to −4.5V. A superimposed differential signal of ±1V yields the final bus voltage swing from 10.5V to −5.5V.
In comparison, EIA-485 specifies a common-mode range from 12V to −7V. Device data sheets commonly define this range as the recommended receiver input voltage range, while specifying the absolute maximum input voltage range from to 14V to −9V.
Industrial networks often experience ground potential differences significantly larger than 7V. For GPDs up to 25V, transceivers with high common-mode input ranges are recommended. GPDs higher than that require bus transceivers to be electrically isolated from their local node circuitry by the means of galvanic isolators in the signal and supply lines (Figure 7).
This method leaves all transceiver grounds floating, thus reducing the GPD down to 0V and the receiver input common-mode voltage to half the receiver supply, VCC2-ISO /2.
The correct method for designing a differential data link is without ground wires. For ground potential differences higher than the specified ±7V in EIA-485, use transceivers with high-common-mode capability, or isolated bus transceivers.
Please join us next time when we will discuss the difference between gain bandwidth product and unity gain bandwidth.
For more information about RS485, see RS485 Design Support . About the Author
Thomas Kugelstadt is a senior systems engineer with Texas Instruments. He is responsible for defining new, high-performance analog products and developing complete system solutions for industrial interfaces with robust transient protection. He is a Graduate Engineer from the Frankfurt University of Applied Science. Thomas can be reached at . Related posts:
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