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How do you go about measuring the actual common mode voltage of a circuit? For example, the RS-485 port of a device. Thanks.

Simple,

Connect one side of your scope (not a meter because this voltage can and usually is transient and can be either AC or DC) to the logic ground and the other side to first one side of the RS485 signal and then to the other wire. The voltage you see is the common mode voltage. The difference between them is the actual signal on the RS485 circuit.

Jerry Miille
Miille Applied Research Co., Inc
http://www.miille.com/
Jerry Miille

Your procedure mentions a third point,i.e., logic ground as the reference. A lot of RS485 devices only has the RS485+/RS485- terminals (plus Shield). Does it mean that the chassis/earth ground is tied to the logic ground? How would an isolated and non/isolated RS485 port factor in? Thanks.

Common mode voltage has no meaning without a reference point. The shield would be a good guess as the reference. If it is completely isolated, the shield would almost have to be it, as shield implies shunting induced currents to a point of fixed potential and low impedance. With a non-isolated port it would be in reference to signal ground, which is generally where the shield goes. In some high Z applications the shield is driven, but not in comms.

Regards
cww

Strictly speaking, the common mode voltage is the average voltage of the two differential signals. The common mode component takes both signals in a positive or negative direction. The important bits usually are that common mode changes should not affect operation and that neither signal should exceed the common mode voltage range, often the supply rails. If you were to use two equal resistors from the + signal to the - signal, the center should give you the common mode voltage referred to ground.

Regards
cww

One of the easiest ways to view a signal like this is to use a dual trace scope of sufficient BW. Connect the gnds of the probes together, and place one probe on one wire and the other probe on the other, since this is a "balanced" line. Then put the scope in the "add" and "invert" mode. This will display the composite signal of both lines "added" together showing you what the signal actually looks like.

Hope this helps.

Excellent response and absolutely correct as far as viewing the common mode signal is concerned!

This is absolutely the way to test for "Common Mode Rejection Ratio" (CMRR) as a component of error to low level analog measurements.

The original question was paired with "RS485" and thus my answer was directed to the measurement of "common mode voltage" as it relates to communication circuits, not low-level analog inputs.

It is amazing to me that many people think that RS485 is a "two wire" type of connection. This is not correct: It is a three wire connection. There needs to be a "common" reference, you could call it a ground but what ever you call it, there is a requirement that limits the common mode voltage that the RS485 drivers and receivers will withstand. It's in the spec.

Have a look at just two web sites that address this in detail:

http://www.bb-elec.com/bb-elec/literature/tech/485appno te.pdf
and
http://www.maxim-ic.com/appnotes.cfm/an_pk/763/

Notice the ground wire!

I am not endorsing either product, just pointing out that there is a limit to the Common Mode Voltage that an RS485 circuit can withstand. You can find many other references on the web. I even have some old TI data book dating from the 1970's that have the same connection drawings and spec's!

Jerry Miille

I agree Jerry. I always used a "cheater" plug on the power supply of the scope to be able to connect and isolate the ground connection of the scope. Also, DC couple the scope to see any shift voltatges. I did like the maxim info also. it was very good. Thanks much.

Hi Jerry,

Very informative links, thanks.

I have used devices(power meters, controllers, RS485-to-LAN gateways, etc.) whose RS485 ports has only 2 pins--and they work. A lot of people thinks its a 2-wire system because a lot of products are sold as such, e.g., Advantech products.
(In the same logic, I suppose RS422 should be wired with 5 wires instead of 4?)

Also, are you saying that interconnecting the third wire(i.e., logic ground) of two or more RS485 devices would ensure that the rated common mode voltage (CMV) will not be exceeded?

Actually, the reason I am asking about CMV is because I am trying to find out if we have exceeded the CMV rating of an old mini-converter(RS232-to-RS485/422, non-isolated I think) attached to a GE9030 plc.

The original setup was: GE9030 SNP port<->mini-converter<->RS232 patch cord<->PC
We upgraded the setup as: GE9030 SNP port<->mini-converter<->RS232 patch cord<->Serial-LAN gway<->Eth. swith<->HMI-PC.
The Serial-LAN gway has a configurable RS232/422/485 port on 1 side and Ethernet on the other.

The new setup worked. However, when we pulled and later returned the DB9 patch cord connector on the Serial-LAN gway end, there was some arc-ing when the DB9 patch cord connector touches the gway connector. After which, the communication is no longer working. We traced the fault to the mini-converter.

What could have caused the arc-ing between the patch cord and gway connectors? The 2 DB9 connectors of the RS232 patch cord are tied thru the shield of the cable--any significance?.


Thanks again.
Oliver Santos

The reason that two wire connections can work is because the logic power supplies at each location are referenced to earth ground many times thru the ground wire in the AC supply. If the earth grounds are relatively good then the common mode voltages stay within the limits and everything is OK. Bringing along the ground wire will insure that the logic supplies at each end are tied together. This is not always possible in the real world and can cause major problems. That is why GE and others make isolated converters.

They have some very complete installation instructions in the 90-30 PLC Installation and Hardware manual (GFK-0356P).

I suspect that if you saw any kind of sparking then you have one or more blown receivers. The transmitters are usually a bit more robust and might have survived but that will do you little good unless they are separate chips.

Your new configuration changed your grounding since the Ethernet cables do not provide a DC path and your original configuration had one via the RS232 connection to the PC.

Jerry Miille

But that isn't the common mode voltage.

Regards
cww