2

If I have a fusebox with about ten/fifteen eastron sdm630 and Orno OR-WE-515 kWh meters with RS485 modbus, can I daisy chain them all to one Waveshare RJ45 modbus server? I'm new with this, but this is called a fieldbus, right?

The end goal is to read all of the devices in Home Assistant to make an energy monitor system.

JRE
  • 67,678
  • 8
  • 104
  • 179
Tim
  • 173
  • 5
  • 1
    Yes, that's exactly how they're intended to be used. Note that you might need some termination at one end of the chain (depends on length, mostly). Although it's excellent, you don't actually need an ethernet-to-rs485 Modbus interface (your Waveshare), you could do this with a USB interface. make sure your Eastron models are ones with modbus (some only have pulses). – jonathanjo Aug 29 '23 at 08:34
  • RJ45 is just a type of connector – jsotola Aug 29 '23 at 09:00
  • 1
    @jonathanjo. Thanks for the heads-up on the termination. I'll read into it. You say, depending on the length. Would finding out be trail and error? I would prefer modbus RTU to TCP, instead of USB. This way I can send the data over the LAN. The server(monitor) is not close to the fuse box. If you (or someone) has a better (or better experience) with a different Master than the Waveshare. Then please let me know. I would prefer a DIN rail mount, to keep in enclosed in the current project. – Tim Aug 29 '23 at 09:10
  • @jsotola RJ45 is not even a type of connector. The connector is called a 8P8C modular connector. RJ45 is just incorrectly used to refer to the connector. – Justme Aug 29 '23 at 09:15
  • 1
    @Tim Yes, it's often very nice to have Modbus/TCP, solves a lot of issues. That particular converter looks good (though I have no experience of it). If it was me, I'd get the POE version with isolation, but it all depends on the rest of your environment. Are all the Modbus meters adjacent to each other? – jonathanjo Aug 29 '23 at 09:37
  • @jonathanjo I'm now also thinking of getting the POE version. When buying the first 3 as a test (not received yet) I thought I need one for each meter... So that would need a lot of PEO ports on the switch. :) Now, with this new knowledge I will return the modules and upgrade it to one single POE version (already went with the isolation version). Yes, all the meters are (almost) adjacent to each other. There might be a few 3phase on different DIN rails, but the signal cables would not be longer than 30cm to connect to next rail/row. – Tim Aug 29 '23 at 09:47

2 Answers2

3

There are many ways to organise this, and the following is just one which is intended to be as easy as possible to do.

  • Yes, Modbus-RTU is a bus. Each node is an RS-485 half-duplex device with three terminals A, B and G (signal ground)
  • Each device gets assigned an 8-bit address, typically by push-buttons on the device
  • There is a limit to the number of devices on a given bus, which is nominally thirty "standard loads", but if you need more than this, check the specifications of your devices and you'll find that most devices are actually 1/4-load or 1/8-load.
  • Modbus-RTU can have various speeds of which 19,200 baud is the required default. In your particular application (kWh meters) there seems no advantage in going faster.
  • Over very short distances (< 1 metre) there is no need to worry about the cabling, just use twisted pair in the obvious way (connect all the As, connect all the Bs, connect all the Gs), making sure there is absolutely no confusion with your mains wiring.
  • Your chosen Modbus-RTU to Modbus/TCP gateway looks perfectly good. Usually it's easier to have a Modbus/TCP in the system than a serial port.
  • Given an equal choice, put the gateway near the meters (because it is often easier to debug a long ethernet cable than a long RS-485 cable; link lights, ping etc)
  • But if necessary, you can have hundreds of metres of RS-485, and it's easier to have a long (500 m) RS-485 cable than an ethernet one
  • Use a POE Modbus gateway if you have suitable ethernet switches
  • Put your automation server wherever convenient

Here are some documents which might be of use

jonathanjo
  • 12,049
  • 3
  • 27
  • 60
  • Thanks, I think I have quite a few pointers now. Some things like "push button on device to get 8-bit address" I have not seen in the product description. But then again, it's not yet delivered. – Tim Aug 29 '23 at 17:36
  • Glad it's of some help. (I just added a link to the Eastron instructions about address settings.) – jonathanjo Aug 29 '23 at 17:40
0

As noted above, the answer is a clear yes, but you need to make sure you set a unique slave address on each unit, so the master can uniquely address them.

colintd
  • 2,018
  • 2
  • 17