Author Topic: RS485 and Current-Loop (C.L.) signal levels - depending number of users  (Read 268 times)


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hello everyone,
I'm dealing with a professional challenge – and I will appreciate your help:

I try to design a schematic circuit so I can define if the communication connected to the line is C.L. or RS485 (for one of our products),
What I need to know, and I can't find any reference about it is:

1.   In RS485: what will be the difference in the signals level (amplitude) between 1 device connected to the line, and lots of devices, say 32 devices?
should the signal decrease? or increase? and why ?
when the maximum should be expected?

2.   Exactly the same questions with Current-Loop communication line.

My main goal is to define the signals level in any cases – a lot of devices and few devices,
To understand when should I expect a maximum signal level and vise-versa – with C.L. and RS485,


Jan Axelson

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From Serial Port Complete, Second Edition:

The total current used by an RS-485 network varies with the impedances of the
components including drivers, cable, receivers, and termination components. A
low output impedance at the driver and a low-impedance cable enable fast
switching and ensure that the receiver sees the largest signal possible. A high
impedance at the receiver decreases the current on the line and thus reduces
power consumption.

When used, termination components can greatly increase the current a line
consumes. Many RS-485 lines have a 120Ω resistor across the differential lines
at each end of the line. The parallel combination of the resistors is 60Ω. The terminations
create a low-resistance path from the driver with a logic-high output,
through the terminations, and into the driver with a logic-low output. On short
cables at slow bit rates, you may be able to eliminate the termination entirely
and greatly reduce power consumption.

With no termination components, the receivers’ input impedance has the greatest
effect on the series resistance of the line. The total input impedance varies
with the number of enabled receivers and their input impedances. Chapter 7
explains how to decide whether to use a termination and if so, what components
to use.

In addition to a 60Ω parallel termination, an RS-485 driver can drive receivers
that draw up to a total of 32 unit loads. TIA-485-A defines a unit load in terms
of current consumption. A receiver equal to one unit load draws no more than a
specified amount of current at input-voltage extremes specified by the standard.

When the received voltage is as much as +12V greater than the receiver’s signal
ground, a receiver equal to 1 unit load consumes no more than 1 mA. When
the received voltage is as much as 7V less than the receiver’s ground, the same
receiver consumes no more than -0.8 mA. To meet this requirement, a receiver
must have an input resistance of at least 12k between each differential input
and the supply voltage or ground, depending on the direction of current flow.

The resistance at each of the two differential inputs of a 1-unit-load receiver is
thus 12k. (This is the resistance from an input to ground or the supply voltage,
not the resistance between the two inputs.) Add a second receiver, and the parallel
resistance of the combination drops to 6k. With receivers equivalent to 32
unit loads, the parallel resistance of the combined inputs is just 375Ω, or slightly
less due to leakage currents.

For current loop:

Because you add the extra current loops in parallel, the power supply’s voltage doesn’t need to change; you only need to make sure it can source the required current. You can mix and match different types of transducers as long as each transducer and power supply’s operating voltage match.


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thank you for the answer,
to make things slightly more clear - i can summarize from your answer that:

1. with appropriate termination resistors - no change should be expected in the signal levels of the RS485 devices? no matter if it's 32 or 4 ?
2. in C.L. configuration, the voltage supposed to remain constant as long as the sufficient current supplying to all of the connected devices ?

am i correct ? 

Jan Axelson

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yes and yes