Weird issue on energy meter's signal with modbus protocol 2

[zenniz]

I'm using Modscan to check the data from my Integra 1630 Energy meter.
Often, i will only receive a register value of 0x00 while occasionally a
good reply. I tried changing the scan rate from 100ms to 5s but nothing
changes. The wire connected from my RS485 to the energy meter is less
than 50m.
Anyone has any idea why this is happening and is there any adjustment i
can do to make the slave send back good reply instead of 0x00?

Here are some readings of the replied data from my Modscan
1st reply - 00
2nd reply - 00
3rd reply - 01 04 04 4c 72 b5 ca bb c8
4th reply - 00
5th reply - 00
6th reply - 00
7th reply - 00
8th reply - 01 44 44 4c 72 b5 ca bb c8
9th reply - 00
10th reply - 00

 

[VEBill]

Probably best to troubleshoot in the analog domain first, inspecting the signal at the receiver end with an oscilloscope - looking for noise or corrupted signal.

 

[zenniz]

I just figured out the problem today. There was no terminal resistor between the twisted pair. That is the reason why i keep getting 0x00. After adding a 120ohms to it, the signal was very smooth.

 

[itsmoked]

zenniz; Glad you got your problem solved. I'd appreciate some info for my data banks. Can you tell me how long your 485 line is and what the baud rate is?

Thanks.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[zenniz]

@itsmoked

Less than 50m(Estimated), didn't really went to measured it. 9600 baud rate.
I did some experiment with the length of the wire without a terminal resistor.
If length of wire is less than 1m = very smooth reading.
If length of wire is 1m or more = occasionally 0x00, it got worst with longer wire.

 

[itsmoked]

Thanks much! Good to know.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[djs]

It sounds like the Modbus software you are using to read the data is not rejecting packets that have parity errors. In a production setting this could be a big no-no.

 

[itsmoked]

djs!! That's what I was wondering. Seemed a strange data error but I didn't think of that.

Keith Cress
kcress -

http://www.flaminsystems.com