RS 485 Communication - Howto - What is it? - Monitoring Hadware

Posted on 2004-11-10
Last Modified: 2007-12-19

This is quite urgent.

I have the oppertunity to develop an app for an engineering company. This company is in the pump building business.
They allready had someone else rig them up a hardware panel with four LCD's for getting readouts from the pump.

These LCD's each sit ontop of their own "unit", which I guesse is responsible for reading the data and displaying it on the LCD.
The Units all have an RS-485 port (to me it looks like a RJ-45 ethernet port...anyway), which is supposed to transmit that data displayed on the LCD through the RS-485 port. My Job will be to read that data from each unit, and draw appropriate graphs and other stuff from the values.

From what I can tell on the net...RS-485 is a kind of a serial network, where you can have 32 devices talking on one twisted pair cable at the same time.

What I need to know is how do I communicate with these devices from let's say VB, and then also what port will I use on my PC...ethernet or serial.

Question by:x50Fade
    LVL 7

    Accepted Solution

    If you're going to use a PC, you'll need an RS232<=>RS485 converter, like the one's sold by B+B Electronics ( This allows you to use the serial com port to talk to the device 'network'.

    I do something similar. The routine is to "poll" each device. Each device must and should have it's own address. You send out the com port a request for data to a specific address. That device answers with the data. You'll have to get the protocol (addressing scheme, ack, naks, data stream, etc) from the device manufacturer.
    LVL 55

    Expert Comment

    by:Jaime Olivares
    >The Units all have an RS-485 port (to me it looks like a RJ-45 ethernet port...anyway)
    Do you mean you have Ethernet to RS-485 converters? If so, which model?
    LVL 2

    Author Comment

    LRHGuy...thanks fot the info.

    I must admit that I do not have the faintest idea on how to do the cabling for this project. How do I connect all four digital panel meters to one UTP cable? Can I use a normal hub?, and do i then convert that utp cable into an RS-232. I understand what you are saying about the "polling", it makes sense, but let me just make sure that I understand correctly....I can talk to any device on that "network", if I have it's address...I send something to it's address, and it replies, right?
    All this info I will have to get from the manufacturer of the digital panel meters, right...i.e. how to talk to it, so that would be my next step?


    No I don't have ethernet to rs485 converters, that's my point.... the ports on the digital panel meters look like they are RJ-45 ethernet ports, but the engineer assured me that they are communicating using RS-485
    LVL 7

    Expert Comment

    Yes, you connect all the cables in parallel. I use a hub or make my own from  a rj45 mount panel, and knock everything down in parallel. In the rj45 "network" all devices are in parallel.

    Depending on the devices, the address may be one or two bytes, i.e. 0-65535. You must get the specs for the device you are attempting to control. You address a packet to it. It replies. All other devices stay quiet until spoken to.
    LVL 2

    Author Comment

    Beautifull !!!

    Thanks LRHGuy

    This all seems to make a lot of sense, but I can see that I still have a long way to go. Thanks for the info and advice.


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