RS485 Dropping Out

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fifty60

fifty60

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Location
USA
There are several machines that are connected together via RS485, and are being controlled over the RS485 network by using Modbus commands.

Everything works well, until one of the machines are power cycled. When this happens, communication is lost to all machines.

My guess is that the communication port impedance is doing something either at startup of the machine, or on shutdown of the machine, that is disrupting the USB to RS485 converter that is common to all chambers.

Anyway to prevent this from happening? I was thinking maybe a power isolator that would be high impedance in the event that power is lost to one of the machines, or something along those lines.
 
R

rlundsrud

Senior Member
Location
chicago, il, USA
Are you using low capacitance cable? Are there any cables that have a different impedance? Any T-taps? Is there a terminating resistor at the end of the line? make sure you check the simple stuff first. You would want 120 ohm lo-cap cable daisy chained with a 120 ohm resistor at the end.

That aside, I would put a scope on the come line (it has to be either a battery powered scope or you would need a differential probe, you can also use two inputs of a line powered scope and do a math subtract). Then shut down the machine and see what happens. You may be getting a discharge pulse from the machine that is bombing your com line when shut down. It would take a pretty big pulse to do this as an rs485 line uses a differential signal, it is very robust.
 
petersonra

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
There are a lot of poorly thought out implementations of modbus that do not deal well with errors and slaves that don't respond when polled. I suggest you contact the vendor of the Modbus master unit and see what they suggest.
 
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mike_kilroy

Senior Member
Location
United States
fifty60 said:
My guess is that the communication port impedance is doing something either at startup of the machine, or on shutdown of the machine, that is disrupting the USB to RS485 converter that is common to all chambers.
Click to expand...

A bunch of important questions have been asked; you really need to answer them before you can expect possible solutions other than guesses.

Add to it: is this 2 wire or 4 wire rs485?

Also, please decode your statement above. Of course I understand your comment on Z, but what is a chamber and then how do you figure these things are related?

Another guess without enough info, I have seen rs485 where it is not a drop but actually goes in and comes out the other side of an active circuit, meaning power down and the loop opens.
 
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GoldDigger

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Placerville, CA, USA
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Retired PV System Designer
If it comes down to the details, it should be possible to get network monitor software for a laptop than can analyze the network traffic and determine the cause of the lockup.
 
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gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
151221-0955 EST

RS485 is a two wire balanced transmission line type system. This can only operate in half-duplex mode, and is usually capable of a multi-drop bus.

RS422 is a four wire type system with two balanced transmission lines. Full-duplex operation is possible, but only one sender at a time is possible.

The 422 and 485 line drivers and receivers have essentially the same electrical specifications. Multi-drop requires a transmitter that can be put into a high impedance output state when not transmitting.

RS232, as normally used, is basically an unbalanced transmission line system, where ground or some common is one of the transmission line wires. RS232 is also a higher impedance system than 422 or 485, but as specified has higher threshold limits.

Common mode voltage, or better described as ground loop voltage, for RS232 creates data noise problems.

For 422 and 485 common mode voltage, the voltage between ground and the transmission line, produces only a small data noise problem. But if the negative or positive limits of common mode voltage are exceed, then there is big trouble. These limits are in the range of possibly 5 to 10 V for typical drivers and receivers. Isolation of drivers and receivers from ground is really required for reliable operation.

.
 
fifty60

fifty60

Senior Member
Location
USA
They loose communications to all of their machines when power is cycled to my machine. It is 2 wire RS485 for my machine, but I am not sure what the other machines are using.

I can daisy chain multiple units of my machine together using my own Modbus software, power cycle, and not lose communications with the other machines that share the RS485 network. I cannot recreate their problem.

I believe this falls into the "poor Modbus implementation" on the end user side, as they are using their own software. There is likely a time-out issue or something along those lines in the software they are using. Is it possible for their master to be waiting for a response from my machine, but the time out is so small that it is overfilling the serial buffer?

I'm not familiar with the recommended software "Device Monitoring Studio", I will look into it some more.
 
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