HC2-S Probe and Modbus

Hi Guys,

Does anyone establish a Modbus communication with HC2-S probe from Rotronic? In the datasheet it says, it has a Modbus interface and also in the software there is selection of Modbus. But the problem is related about the register of values that I should read in Modbus. There is nothing about that registers just a unit address and no register address. My question is how can I read the data registers without any addresses? By the way, I think may be is related to the protocol which I think is Modbus ASCII and not a Modbus RTU. Does the addressing in both protocol are the same?
 
Good Luck. I was threatened with having to implement this about a year ago and was relieved when the project died because their 3 pages on Modbus in the comm manual did not inspire confidence.

From notes I have, I believe that the HC2 probe plugs into a Rotronics 'transmitter' that interfaces to the real world. According to their web site, the HygroFlex HF4 'transmitter' claims to have a Modbus ASCII comm interface. Maybe other 'transmitters' do, too.

The HF4 manual states:
"Modbus: the Modbus protocol available with the HF4 is limited to reading measurement data from the HF4. Functions such as device configuration, humidity and temperature adjustment, etc. are not supported by the Modbus protocol."

The Rotronics site search engine does not return a hit for their The E-M-AC3000-CP_20 communications document. Somehow I got a copy, probably from a rep. It has 3 pages on ASCII Modbus.

Note that you need Rotronics proprietary HW4 software just to enable the Modbus protocol (pg 3):
- The HW4 software is required to set the AirChip 3000 device to use any protocol other than RO-ASCII
- When communicating with the HW4 software the AirChip 3000 device automatically switches to the RO-ASCII protocol (ROTRONIC standard).
- After using HW4 to configure the AirChip 3000 device to use one of the available protocols power down the device. When the device is powered up, it automatically starts using the selected communication protocol as long as it is not communicating with the HW4 software.

There are only 3 variables available via Modbus ASCII
- Humidity
- Temperature
- Calculation
each of which is enabled or disabled in the HW4 software Modbus setup page.

Their example ASCII frame format states that the Start Register address will be ignored, as well as the number of registers (will also be ignored). So I assume that the reply provides the number of variables specified by the HW4 software, regardless of what is requested.

It also states for the LRC checksum "(optional in data request, 2 characters, will be ignored)". I assume that means it does not check the LRC. Hopefully it generates an LRC for the reply, but who knows exactly what 'will be ignored' means?

Also gleaned from the comm manual:

Function Code 03 Read Holding Registers is the only supported Function Code.

There is no address register list.

I was prepared to start polling at (4)0000 and work upwards until I got a hit.

Please let us know if you actually manage to get a value out of one of these devices.
 
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