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BRX/Do-More Relay Module as Low Level Source Switch

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  • BRX/Do-More Relay Module as Low Level Source Switch

    I have an odd setup that I'm trying to get fancy and solve to prevent human errors. I have several flowmeters with a mix of magnetic and RF pickups with appropriate amplifiers/signal conditioners. We constantly change them out and switch between the two types of pickups/amplifiers as our needs require for accuracy and such. Right now I have terminal blocks and connectors and it is a wiring mess and my technician is constantly in and out of my panel to switch connectors and amplifiers. What I am hoping to achieve is a cleaner and more robust setup in my new facility that no longer requires access into my panel using a relay module as a switch. My plan is to have one flowmeter cable split into both amplifier types and the output of each amp connect to a load side of the relay module, then use the module common as an output to feed the appropriate amplifier output the HSIO module. This would use the relay module "backwards" as a form of switch and my SCADA program would allow the technician to select the appropriate amplifier for a given flowmeter. Thus the technician only ever has a single cable to worry about and a checkbox on a screen.

    Is this an acceptable use of a relay module? Or is there better module/option that someone can recommend?

    I have spare outputs on a BX-12TR and room for additional expansion modules on the rack with my BRX setup, and I can add whatever modules to my DoMore DL205 setup as needed. The outputs of the amplifiers are both square wave pulses, 0-24Vdc pk-pk, 0-15kHz if that matters.

  • #2
    The specifications for the BX-xxTR modules state a 0.1mA "minimum output current" which is fairly low. However the module does have a snubber circuit across the output with a "maximum leakage current" of 1uA, which is also pretty low, but for some sorts of instrumentation it still might be significant, though leakage per se may not be a problem in your case with what I'm guessing is essentially frequency modulation, on the other hand the "C" in RC network might not be so good with kilohertz signals

    If it were me i'd consider using a transistor output module driving gold plated contact small signal relays without snubbers.
    (I mean snubbers across the contacts, one should still put something across the relay coil)
    Last edited by Tinker; 02-12-2020, 09:59 PM. Reason: addition

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    • #3
      Could you add another HSIO module and wire one sensor to one, and the other to the new module? Then, in the program, just pay attention to the proper input based on the checkbox on the screen. More expensive this way, I suppose, but there's no relays or other interfacing modules to worry about wiring correctly.

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      • #4
        That was my fear running a relay module. Funny, as this goes I am making many upgrades to my facility and after posting this I realized I am forced into getting another HSIO simply due to required number of outputs for stepper motors. With three HSIO/CTRIO's I will have access to 12 edge timers and 6 stepper motor pulse/dir outputs which is enough to handle my flowmeters without trying anything outside my comfort zone. I have no experience with the BRX hardware so I am basing most my knowledge off the DL205 DoMore. Can anyone confirm if the BRX HSIO can handle more than four edge timer inputs and two stepper motor outputs? The spec sheet is a little confusing because it says 8 inputs but only "Up to 4" functions.

        Thanks guys.

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        • #5
          "The spec sheet is a little confusing because it says 8 inputs but only "Up to 4" functions."

          I don't know what the designers thinking was, but consider that a quadrature counter would need 2 inputs so one could only have a maximum of 4 counters in that case and maybe they decided it wasn't worth the bother to allow more functions for single input counters. Also the software is free, but they make a profit on selling you modules, just be glad they give you four, it could have been one or two. The significantly more expensive H2-CTRIO only has two channels

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          • #6
            Originally posted by jvian View Post
            Can anyone confirm if the BRX HSIO can handle more than four edge timer inputs and two stepper motor outputs? The spec sheet is a little confusing because it says 8 inputs but only "Up to 4" functions.

            Thanks guys.
            There are only 4 input functions per HSIO module:


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            If you have enough inputs with the second HSIO you are going to get then the best solution is to use that and have software logic switch between the different inputs.
            If you still dont have enough inputs, instead of a relay module you could use some DIN rail mounted double-throw relays and do exactly like you thought in your first post, only with external relays instead of a module. External relays would probably be better for this situation anyway since you can change them out when a relay goes bad.
            You could use SPDT relays, one for each sensor pair, or half as many DPDT relays and put two sensors of each type onto a single relay. Whether that would save space and cost or not I dont know, youd have to look all that up.

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            • #7
              Originally posted by jvian View Post
              The spec sheet is a little confusing because it says 8 inputs but only "Up to 4" functions.
              Originally posted by Tinker View Post
              I don't know what the designers thinking was...
              Let me help you with that.


              There are 8 fast inputs and 8 fast outputs. The physical inputs can be assigned to any logical function input, and a single physical input can be assigned to more than one logical function input. The physical outputs can be assigned to any logical function output, but can only be assigned one place.

              There are 4 counter/timers functions. Each counter/timer can use up to 5 inputs.

              There are 4 PWM functions. Each PWM uses 1 output.

              There are 4 TDO functions. Each TDO uses 1 output.

              There are 3 physical motion axes, and 1 virtual. Each physical axis uses 2 outputs.

              There are 4 input event interrupt triggers. Each trigger can use up to 8 inputs.

              There are 4 match register interrupt triggers (looks at counter/timer accumulators or axis position registers).

              There are 4 microsecond timer interrupt triggers.

              So yes, there are 8 inputs, but only 4 counter/timer functions. The basis for the function limitation is FPGA capacity. In a perfect world there would have been 8 counter/timers, but there are always trade-offs between price and features.
              Last edited by BobO; 02-13-2020, 02:20 PM.

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              • #8
                Thank you again for the replies. I will be adding the appropriate extra CTRIO/HSIO module and handle this via software. Using three modules I will have enough inputs and outputs for everything I need include one spare input and output for future growth.

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