Using the EKM iSerial TCP/IP Ethernet to RS-485 Converter

[dpapathanasiou]

We're using the EKM iSerial v.2 TCP/IP Ethernet to RS-485 Serial Converter(1) to communicate with 8 daisy-chained Modbus RTU devices over ethernet.

The devices in question are three-phase NF29 digital smart meters(2) by Trinity of India.

As described in this blog post(3), the read addresses begin at 3000 in three-phase mode.

We first tried using libmodbus(4) in tcp mode.

This is what was transmitted when we tried to read 2 bytes from meter number 5, starting at address 3000 (2999 as zero-indexed):

[00][01][00][00][00][06][05][03][0B][B7][00][02]

The response was:

Waiting for a confirmation...
[03][04][00][00][00][00][BF][F3][05]
Invalid TID received 0x304 (not 0x1)
Invalid data

Since we weren't sure what kind of transformation the EKM converter might be doing to the message, we also tried creating the byte stream expected in RTU mode, including the checksum, and sending that over tcp, to the ip and port where the EKM converter is running.

Here's what we sent:

[05][03][0b][b7][00][02][77][8d]

The very first response was:

read 1 bytes
05

When we tried the same request a second time (we were closing the tcp socket after every request), we got this:

read 4 bytes
03040000

And then each subsequent request of the same sequence of bytes (i.e., [05][03][0b][b7][00][02][77][8d]) resulted in the same response from the EKM converter, appended to the previous responses:

read 8 bytes
030400000000bff3

read 13 bytes
030400000000bff30503040000
read 17 bytes
030400000000bff305030400000000bff3

Basically, we kept getting the same reponse pattern from the EKM converter -- [03][04][00][00][00][00][BF][F3][05] -- no matter what we sent (later we tried different meter numbers, start addresses and number of bytes to read, but always the same response).

So that's why we started thinking that we have misconfigured the EKM converter somehow.

We are certain the wiring is correct, and we are using the default ip address and data control port settings (192.168.1.125 and 5000 respectively) to coomunicate with the EKM converter.

Is it a configuration issue, or is the EKM converter not compatible with non-EKM meters?


(1) http://www.ekmmetering.com/ekm-metering ... ction.html
(2) http://www.trinityenergy.co.in/products/nf29.htm
(3) http://www.automation-talk.info/2010/12 ... eters.html
(4) http://libmodbus.org/

[Jameson]

Im not going to be able to help you much with reading a 3rd party kWh meter.

But I can say that the iSerial will be meter agnostic. It is not smart and does not care what kind of data you send to it or it receives. It just translates TCP signals into RS485 signals.

If it were me, I would sniff the output of the iSerial. I would also sniff the output of an RS485 converter that you know works with your meters. If they are the same then it should work.

Hope this helps.

[dpapathanasiou]

Hi Jameson,

Thanks very much for your prompt reply.

We're going to try that next; we don't have a way to talk to the meters directly over RS485, but we just ordered an RS485 to USB converter (from EKM, actually), and we're going to confirm that our understanding of the Trinity access protocol is correct.

In the meantime, I wanted to understand what you meant by " It just translates TCP signals into RS485 signals".

Does that mean that if the target device expects modbus RTU formatted messages we need to send that way?

Or is it doing any translation of modbus TCP into modbus RTU and vice-versa? (i.e., since the modbus TCP message is just the RTU message but without the checksum and additional headers for tcp/ip)

[Jameson]

Does that mean that if the target device expects modbus RTU formatted messages we need to send that way?

Yes, whatever you send to the virtual COM port (in HEX), the iSerial will transmit as RS485 HEX, and whatever your device responds with, the iSerial will provide back.

That is about all I can help you with.