I am trying to interface two systems through RS422 link. There are onl four lines connected between them (tx+/- & Rx+/-) and no ground Independently both systems work fine but when interconnected and i try t communicate with each other there are some problems. initially maste units Tx lines have been set to a known state (3.6v app). if i transmit bytes from master unit only 3 bytes are received at the slave unit, thoug when captured the waveform in CRO shows all the 4 bytes.iam glowin separate LED's for each byte received.the 4th byte LED doesnt glow at al but when the master is powered off, then the fourth byte LED glows. wha could be the problem.
This is quite OK as long as there is a termination resistor (typically
100..120 ohms) between the Tx+ and Tx- lines and also between the Rx+ and Rx- lines. The ground connection is not needed, as the RS-422 works as a balanced current loop.
Wrong -- take a look at the receiver's spec for max common-mode voltage. It's usually only 8-12V. You've got to have a common ground to make sure you don't exceed the receiver common-mode voltage spec.
Wrong -- it's a differential-mode voltage signal. It's not a current loop signal.
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Even when they are, I've had problems unless there's a 5th wire. In theory they should float together and work fine, but in my experience you can't count on it.
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Yep, gotta' have a reference for the signals, even if they are "differential", at least in this case. You hit it on the head when you made the comparison to a current loop. I'll bet you dimes to doughnuts that once a good common is run between the units all the problems go away. At the very least that is one more thing to mark off the checklist.
And what would a flimsy signal ground wire help to reduce the common mode voltage in mains powered systems ?
To understand the situation you have to look how different earthing systems work e.g.
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In a TN-S system with separate protective earth (PE) and neutral (N) no current is flowing in normal situations in the PE conductor and hence all device cases (and the RS-422 transceivers are usually connected to the case) are at the same potential. In this situation using a separate signal ground does not make a difference.
For TN-C-S systems, the situation is the same, provided that the two communicating devices A and B are after the same split point where the PEN conductor is split into PE and N conductors.
Looking at the TN-C system, in which there is only a single PEN conductor carrying the neutral current as well as connecting the equipment case (and usually the RS-422 ground reference), the situation is more complicated. The current flowing in the PEN conductor (typically several amps in a non-balanced three phase system) will cause a voltage drop between mains sockets A and B and hence also a reference potential difference in the RS-422 transceivers for equipment A and B, which can be several volts.
If we connect a signal ground (SG) between equipment A and B, this will create an additional current path parallel to the PEN conductor and the neutral wire current is shared inversely proportional to the resistance of the PEN conductor and the SG conductor.
In order to significantly reduce the ground potential difference between equipments A and B, the SG conductor would have to carry a significant part of the PEN current (say 1/2) thus the SG conductor cross section area should be similar to the PEN conductor.
If you make the SG conductor that thick there may flow several amps of
50/60 Hz neutral current and all their harmonics and other noise currents through the SG wire as well as flow within the equipment (often through narrow PCB tracks) into the device case, which can have harmful effects or even burn a narrow PCB track.
In a TN-C system, the separate SG wire does not solve any problems, but creates some new problems. If the ground potential difference is larger than the RS-422 transceiver common mode voltage range, the only alternative is to use a floating (galvanic isolation) system. In a floating system, the "fail-safe" pull -up/down resistors will pull the internal signal ground into the middle of the transceiver common mode voltage range, so there is no need for a SG wire.
You can analyze the RS-422 system as a voltage driven system, as a bipolar current loop or as a transmission line with impedance matching to the transmission line.
To understand why the SG is not required, it is easiest to look at as a current loop. The standard requires that the receiver voltage is at least +/-200 mV and assuming there is a 100 resistor at the receiver, at least +/- 2 mA will flow in this current loop from Tx+, through the termination resistor (at the receiver end) and back to Tx-. With a 5 V transceiver, the maximum current is below +/-50 mA.
Does the unipolar 0/20 mA current loop need a separate signal ground ? Why would the bipolar RS-422 current loop need a signal ground ?
I've found the same thing. Ya gotta have a common ground for reliable operation. It has always seemed to me that it should work with twisted pair shielded cable grounded at one end - but it doesn't in practice : (
Why not run the signal lines into an opto? That's even the _required_ configuration for the RS-422 receivers in the NMEA 0183 standard. Data rate could be an issue, optos tend to be slow; 0183 runs at 4800 baud so it's pretty forgiving in this regard.
I'm not really sure what you're asking. In the systems I'm familiar with, RS-422 drivers and receivers are usually galvanically isolated from mains power, and signal ground isn't always connected to either case or earth ground.
Analyze all you want. I've seen a lots of RS-485 and RS-422 installations, and not connecting a signal ground often makes them fail.
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Let me rephrase, in a TN-C electric distribution system, what difference does a (say 0,3 mm) signal ground conductor have when connected in parallel with a (say 1 mm) diameter PEN conductor ?
Regardless of the presence of the signal ground, the voltage difference along the PEN wire is more or less the same.
As a rule of thumb, I have used the following principle:
RS-232 is OK when connecting equipment in the same rack (such as various modems and current loop converters),
differential RS-422/485 is OK for galvanic connections in the same equipment room,
galvanic isolation (500 V/2500 V) is OK within a building.
However, if you are connecting equipment in different buildings, an optical fiber should be used, since during a thunderbolt and due to the grounding resistance, the ground potential might have s several kilovolt peak relative to the surrounding countryside.
Using galvanic isolation between sites A and B, I have never noticed any RS-422/486 problems due to the removal of the SG connector. However, the loop current did cause some problems when the connection was installed.
Unless you really understand what you are doing, using galvanic isolation on the equipment level _might_ still cause a lot of problems at the system level, when cable shields are connected to points, in which they should not be connected.
As you can see, it has to be pretty `leaky` for the signals to go outside the CM range of +- 7VDC.
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The + and - outputs are not isolated from each other. ;-) Best regards, Spehro Pefhany
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I don't want to be argumentative, but you have make a lot of assumptions in your discussion. For example, you assume that the chassis of both units "are at the same potential". If that were true, the OPs circuit should be working. If they are not at the same potential, then the reason for that needs to be identified. Your discussion makes assumptions for why that is and makes the assumption that the chassis and/or the electronics are grounded to the power ground. Many systems are isolated just for this reason, to prevent power supply current from changing the ground level of the circuit.
In your discussion of RS-422 as a current loop without a ground, you don't mention what voltage is present on the conductors relative to the associated ground, only the current through them. If you ignore this voltage, you run the risk of blowing out the chips or at least making them work improperly. A common ground must be established between an RS-422 driver and receiver. How to best do this depends on the equipment and the grounding method used. But to say the common ground conductor is not required is a significant oversimplification.
You need to explain better how the power connection is made between them and how the RS-422 chips are grounded inside the separate systems. Is there already a common ground between the two systems and what current is flowing through this ground? Is there enough current to produce a significant voltage difference in grounds?
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