I am looking for a simple device to do the following, preferably in one device if one exists:

Poll one MODBUS register continously and depending on the value of this register open or close a relais.

I am looking for suggestions for which hardware to use. Obviously I am looking for a solution that does not cost to much but works reliably.
(e.g. using s small ESP32 board with a relais attached to an I/O pin would be very cheap, but it is not really a professional solution so this is not really and option for me, unless there is such a solution complete in a box that "just works"...).
Any suggestions?

 

See our Promux modbus RTU RS-485 or TCP ethernet modules.
Work extremely reliably, simple and very rugged. Hardware has automotive electronics specifications.
See: https://cascade.net/en/modular-plcs/
See all modules in the brochures.
RTU: see the PM4RO with relay outputs on page 49
TCP: see the PT4RO with relay outputs on page 47

 

Can you clarify your requirements?

"Poll one MODBUS register continously" seems to imply you're looking for a Modbus client/master device as opposed to a server/slave device. Is this correct?

What version of Modbus do you need (serial Modbus, such as Modbus RTU or Modbus ASCII, or Ethernet Modbus, such as Modbus/TCP)?

What type of circuit will the relay be operating? What are the load voltage and current requirements? Can the relay be solid-state or must it be a traditional contactor relay?

 

Can you clarify your requirements?

"Poll one MODBUS register continously" seems to imply you're looking for a Modbus client/master device as opposed to a server/slave device. Is this correct?

What version of Modbus do you need (serial Modbus, such as Modbus RTU or Modbus ASCII, or Ethernet Modbus, such as Modbus/TCP)?

What type of circuit will the relay be operating? What are the load voltage and current requirements? Can the relay be solid-state or must it be a traditional contactor relay?

We have a Photovoltaic Inverter and need to control an external relais depending on the state of the inverter.
So the MODBUS device IMHO should be a master to be able to do the polling. But the relais that we want to control would be a slave. My first question would be if we can find this in one device. I assume the normal MODBUS relais which are available can not poll a register themselves...
We need to automate some things in the electrical system of the client using a larger relais (we are talking 400V 25kW 3Phase). I assume that the MODBUS relais client is a small device which would drive a 40-63A 3P power relais.

Most likely we need/should do MODBUS RTU. We probably also could do TCP but the inverter is connected via WiFi and we don't have Ethernet near by.

 

The modbus slave I talked about has a register where you should write the state of the relays in.
It is a low power contact, so drive another bigger relay using this contact.

 

The modbus slave I talked about has a register where you should write the state of the relays in.
It is a low power contact, so drive another bigger relay using this contact.

Yes, but to write the state I would need also a master, since the inverter is also only a slave...

 

of course.

 

You may be able to use the ICC Mirius gateway in conjunction with larger relays that accommodate the 400V 25kW 3Phase requirement.
http://www.iccdesigns.com/protocol-gateways/66-mirius.html

The Mirius can be configured as a Modbus RTU Master and has 2 dedicated I/O channels that can be configured as solid-state relay outputs (open-drain style, low-side switching). Each output can switch loads of up to 250mA and supports external biasing of up to 24VDC. If your relay does not include a flywheel diode, you would need to add one externally. Here is an example of what the circuit would look like (note that the transistor and resistors are already inside the Mirius, so you just need the relay and flywheel diode):
https://www.electronics-tutorials.ws/blog/relay-switch-circuit.html

In addition to the above, the Mirius could also expose the Modbus data from your system to a BMS supporting other serial protocols, such as BACnet MS/TP.