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RS485 polarization + common ground

Former Member

Hi there Everybody.

 

I've had few problems with my devices with embedded RS485 tranciver.

 

I've constructed a USB<>RS485 Half Duplex converter witch is based on FT232R chip, and SN75176 (almost the same as MAX485). Well as the data sheet said, the RS485 bus should have terminating resistors on each end. Of course I've added them (120Ohm resistors). I've tested the devices in my workshop, it worked just fine without any problems.

 

The problems however started to aper after installing the devices in the working place. There are a lot of errors in comunication, a lot of dummy data. The reason for that is probably that when there is no transition on the  line (all transmitters are off) the bus is not droved by any signal and  is in HiZ. I've decided that the problem is in lack of polarization resistors (2x 510Ohm pull-up/pull-down on A and B).

 

Well the question here is pretty simple and short. Should I polarize the bus in one place (e.g. middle), where the terminators are (on ends in 2 places) or in each device?

 

Well for me its pretty simple, that the device should be polarized in one place (maybe two). I've read somewhere that all the devices should have those resistors witch I think is wrong thinking. From intuition the pull-ups/pull-downs will be to strong. This can be counted with a parallel resistors model. I really think that when there are more devices on the network the transition will be worser quality. What are you're thoughts on that?

 

The second thing I've wanted to ask is what about the device grounding. Should the devices have common ground witch goes along with the whole bus? Or should they be grounded from PE cable from the sockets? Well I've read somewhere that if there is no common ground and the bus is long the communication could not work. This is caused by big potential differential between places where the devices are.

 

I've would be pleased to here you're place on those 2 issues. Please mark in You're responses if its a theory or did You used it in practice.

 

Sorry for my bad English.

Regards

H-D

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  • Well the question here is pretty simple and short. Should I polarize the bus in one place (e.g. middle), where the terminators are (on ends in 2 places) or in each device?

     

    Well for me its pretty simple, that the device should be polarized in one place (maybe two). I've read somewhere that all the devices should have those resistors witch I think is wrong thinking. From intuition the pull-ups/pull-downs will be to strong. This can be counted with a parallel resistors model. I really think that when there are more devices on the network the transition will be worser quality. What are you're thoughts on that?

     

    See Doug's answer with the pull-up on the non-inverting and pull-down on the inverting.

    @ , even if it's differential, it's not supposed to be floating. You'll have to be sure that the inputs are at defined  (differential) levels.

     

    The second thing I've wanted to ask is what about the device grounding. Should the devices have common ground witch goes along with the whole bus? Or should they be grounded from PE cable from the sockets? Well I've read somewhere that if there is no common ground and the bus is long the communication could not work. This is caused by big potential differential between places where the devices are.

     

    You'll need to add a reference wire, to connect the reference of one system to the others'. There's only a limited amount of common mode voltage that a RS485 receiver can suppress / survive. You need to take care that both drivers' levels do net exceed the maximum common mode voltage, so they'll have to have their local GNDs connected to each other. You might use PE for this, but be sure that you understand what you're doing. Is the PE also the local GND of both systems? If you're doing anything related to human safety, be very much aware of what connections you are making, and how currents can flow through your system.

  • in reply to vsluiter

    I stand corrected.

     

    Our RS485 used isolated supplies, and together with a document from the DOT about the issues they had re-enforced the 'floating' belief.

     

    This document is worth reading and explains the various parts and problems

    http://ftp1.digi.com/support/documentation/90000641_a.pdf

    I noted they suggest the use of a 100ohm resistor to the PE to help prevent earth loops.

     

     

    Thanks

    mark

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