Modbus RTU Slave adress 253

rwagn · Post by **rwagn** » 27 Mar 2020, 17:46

Hi everyone,

I would like to talk to a sensor via modes (rs485).

I do hava a RevPi connect + as well as a USB RS485 adapter from RevPi Kunbus shop.

The sensor I would like to use is a soil moisture sensor SMT100 (http://www.truebner.de/sites/default/files/AN002.pdf) with the Modbus broadcast dress 253.

I have tested the communication using the USB adapter I have bought from the shop with my Macbook and a terminal software and it worked. So far so good.

Connecting everything with the RevPi it went out that nothing happens

I have read through all tutorials, youtube videos and blog entries and modified the paths (e.g. /dev/ttyRS485, /dev/ttyUSB2 for the adapter).
Unfortunately, nothing worked out.

Therefore, I have to say that I need some guidance from your side. It seems that the Modbus address space is limited to 200?!?

I have tried to get a direct feedback using you hotline but due to CORONA without any luck.

I would like to ask if there is any chance that I could get some sort of assistance in this forum to solve my problem. I am sure it is a simple mistake from my side, perhaps a wrong setting. But I do not know what else I can do at the moment….

Thanks Robert

Post by **dirk** » 30 Mar 2020, 13:43

Hi Robert, I am a bit confused after a look at the user manual. First there is a fixed broadcast address of 253 but there is also an example how the modbus sensor address may be changed:

: UserManual; 2020-03-30 13_38_44-Window.jpg (69.94 KiB) Viewed 2646 times

So please try to change the address to a lower value. There is a windows software required but with "bootcamp" you should be able to start it with your mac.
Mandwhile I'll try to figure out which addressed may be configured with the RevPi Modbus,

Post by **dirk** » 31 Mar 2020, 13:56

Robert, have a look at the chapter 2.2 of the Modbus specification here http://www.modbus.org/docs/Modbus_over_serial_line_V1_02.pdf

: 2020-03-31 13_52_21-Window.png (25.07 KiB) Viewed 2627 times

Address 253 is in the reserved address area. But technically there is not a limitation in PiCtory to prevent using this adresses.

Post by **dirk** » 01 Apr 2020, 10:59

Robert, we have tested that there is not limitation of the Modbus Slave address in PiCtory so the slave address setting of 253 is possible.