Wingdings and Troubleshooting Modbus Comms

The common thread of all support calls is troubleshooting a protocol. There can be 👈 at RTA for when issues arise, and when the issue is resolved and it’s not RTA related, there is a lighthearted 👉 back at ya’, then a 👍 (ok, technically not a finger), but the other finger (censored) is reserved for RTA’s Engineering team when there is a software bug out in the field. To the team’s defense, name one laptop, smart phone or gaming system that doesn’t have technical hardware/software flaws and the annoying software updates to fix them. The 👍 is for the patience most customers have for us trying to understand and resolve their issues. RTA prides itself in its support and ongoing training for its customers. We love to help, and there’s no better satisfaction than getting a 👌 or 👋 from you.

But why cross your fingers (couldn’t find a Wingding for that one) before your next install? Being aware of these common Modbus comms issues beforehand will keep you from becoming a ticking 💣.

  1. Comms RTU Master/Slave
    • Verify Baud Rate, Stop Bits, Data Bits and Parity.
    • Swap wires: Rule is with RS485: TX(A)- ➔ RTA TX- and TX(B)+ ➔ RTA TX+. You’d be surprise how many support calls get resolved by this.
    • Slave Address: With RS485 RTU you can daisy chain up to 32 Slave devices (don’t forget termination). For example: RTA is the Modbus RTU Master, make sure that all Slave device addresses matches what the master is trying to connect to. Most of these Slave devices will have a display on them and it’s as simple as navigating through the menu.
    • If using a Modbus PLC, ensure the correct relationship is selected.
  2. Comms TCP/IP Client/Server
    • Make sure you can ping the device, verify the IP Address and Unit ID is correct.
    • RTA uses the Modbus default port number of 502
    • Are we all on the same network? Try a different cat5 cable.
    • If using a Modbus PLC ensure the correct relationship is selected.
  3. Is there power to the RTA product? (YES this has happened before.)
    • Integrator: “blah blah blah 30 years’ experience, I know how to wire up a panel.”
      RTA: “Is there power to the unit?”
      Integrator: 💣.

Tools from Win-Tech like ModScan and ModSim are great for troubleshooting. ModScan will make your PC an RTU Master or TCP/IP Client that connects to RTU Slaves or TCP/IP Server devices. ModSim will make your PC an RTU Slave or TCP/IP Server that allows connections from a RTU Master or TCP/IP Client. Both tools are free to use for the first 3 minutes, then you must close out and relaunch. The unlocked versions cost less than $100 each.

A RS485 to USB convertor that’s great for troubleshooting Modbus RS485 is the B+B Smartworx USOPTL4. It runs about $125.

Or, maybe Modbus isn’t the right protocol in general. Our fearless leader has written many blogs on Modbus, and this blog may give you some insight into whether Modbus is or is not the right protocol for your project.

However, Modbus is usually inherited, and there are tens of thousands of Modbus devices (Modbus RTU and Modbus TCP) installed, and they aren’t going anywhere fast, so diffuse your 💣 by taking the suggestions above, and hopefully your next install will give you ✌ and a ☺.