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  • Mike@Forshock
    started a topic Baud rates and Modbus

    Baud rates and Modbus

    So we have tried many ways using many products. (modbus IAP devices) so we are reaching out to others in the field.

    We have a legacy system running an ASCII type protocol, but the Baud rates is 1200, e, 7, 2. Not very modbus compatible...

    We have tried a Modbus IAP with dual serials connections that tunneled each port. We have 1 port set as 1200, e, 7, 2 and the other as 1200, n, 8, 1 (PLC side) and many other variations. So long as we have 7 data bits, we get CRC fails. If we set both ports as 8 data bits, no issues at all.

    We are trying to convert these old systems to PLC, but replacing dozens of units at one time is not feasible or economical.
    We "could" run two radios at the master (one legacy, one modbus) but see previous sentence, cost.

    Anyone have any other ideas?
    Your assistance in truly appreciated.

  • Mike@Forshock
    replied
    Also on the 900 system, we have used them with good success. One site in particular though has a lot of fresnel zone problems (terrain, trees and buildings). the 450Mhz isnt so bothered by them and has much less interference.

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  • Mike@Forshock
    replied
    Very close, the only real duplicate radio will be at the master. As a site is converted, it would be reconfigured for the PLC.

    Now as I am sitting here, I am testing another idea: DirectNet.
    It works when I have the data as 4800,e,7,2 converted to 9600,n,8,1. But dropping port speed down any further results in "No response from secondary unit" error. So the 1200 bps problem remains with that idea...

    The general idea here is that we can convert a few sites per year (small budgets) and while we may spend more in the long run it is easier to afford than all up front. The "credit card" theory is what I call it.

    Again, this is for already small systems; larger systems can usually afford the dual-radio system concept.

    As is, if this works on DirectNet with the current RTUs, we have an additional hardware device that costs <$400 each site. So for the small systems with 3,4 even 5 sites that is less than an additional radio and supporting items and licensing.

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  • ControlsGuy
    replied
    So, you're saying the slave radio will serve multiple RTU's, whether the 1200-e-7-2 devices you have now, or new PLC's with Modbus RTU, and you don't want to change an entire site at the same time. Do I have that right?

    I had been assuming only one RTU per remote and thinking to add one radio at the one master end.

    But even if you end up with two radios at the slave end, that one radio gets reused. Once you finish converting that remote site, then you're back to one radio and can use the previously second one as the second one at the next site you want to convert, till that site is done, and so on.

    Besides, what's the range? Is it such that you have to use licensed radios? Unlicensed 900MHz work well in my experience and go a long ways.

    Also, if a Modbus ASCII-RTU converter gets you out of the fix, consider rolling your own with a little PLC. Some of them have really cheap serial ports, and you might get it done for less than $500.

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  • Mike@Forshock
    replied
    I agree, but some of these sites only have 2-6 units.
    So factor in FCC licensing (new frequency), and supporting items for a new radio install. I would rather do that, in fact for the larger systems we are doing just that.

    The biggest problem is that 7 data bits...

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  • ControlsGuy
    replied
    How many remote sites?

    Would one single new radio at the master end work? So two radios at the master end, and one at each remote? If there's a decent number of remotes, that doesn't seem like an unreasonable expense.

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  • Mike@Forshock
    replied
    Existing systems
    ASCII Protocol (very similar to MODBUS ASCII, VERY similar) running over serial radio at 1200,e,7,2.

    We would like to be able to transition in, in single units PLCs (AD units). They only support Modbus RTU (or TCP with ECOM).

    While we have found "off the shelf" Modbus ASCII to RTU convertors, these are typically well over $1k which at that point we may as well buy a new radio (~$1700). Additionally 7,e,2 is NOT standard Modbus ASCII.

    Leave a comment:


  • ControlsGuy
    replied
    Could you explain the overall system architecture and goals in more detail?

    Leave a comment:

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