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Communication LED in MODBUS
Is Communication LED necessary in MODBUS?

I'm working on a commercial project. It supports MODBUS RTU/ASCII and Master/Slave mode.

There is no communication LED in the current plan. Only the status LED indicates power to the device.

Should communications LED be added?

By Bob Peterson on 14 June, 2018 - 10:44 pm

There is no requirement that it be there. Usually it means that there is some kind of valid modbus transaction. It's handy to show that your modbus link is actually working.

I use a specific brand and model serial server (485/Ethernet) even though I hate the non-removeable wiring terminals because it has blinking LEDs that have proven to valuable in getting the serial side operational.

There's a number of issues that make 485 more troublesome than Ethernet, and getting some indication of 485 driver activity is helpful.

You might consider trading the cost of support time in absence of LED(s) vs the cost of including LEDs. How many support calls before you realize the value of having an LED?

Thanks for advice, Bob Peterson and David_2.

If possible, I will add COM LED.

Hi...i am a new user here.

As per my knowledge the TX LED should normally blink as soon as the Modbus send function is active. Check the call of your send block and validate how it is called, and that it does address the correct CP address. The RX LED will only blink if the CP detects a correct electrical connection with the communication partner. How have you setup the Modbus RS485 cable?