MODBUS Binding with SMA inverter missing lower byte

Setup, Configuration and Use Installation
16

Continuing the discussion from MODBUS Binding with SMA inverter missing lower byte:

I have done some additional tests to investigate the “missing lower byte” issue.

Test conditions:

  • To exclude a too high load as possible cause of problems, I used only one polling Thing (Current Power register 30775, length = 2, polling rate set to 60000 ms

  • Wireshark was used for data capturing with its “packets reassembly feature” turned off

  • For test 1 and test 3, Wireshark was running on the RaspBerry PI (host of OPENHAB), the display was redirected (PUTTY / XMING) to an INTEL-NUC Windows computer.

  • For test 2, Wireshark and the Radzio! Modbus Master Simulator were running on the INTEL-NUC Windows computer.

Tests performed:

  • SunnyBoy SB2500TLST-21 inverter queried from OPENHAB

  • SunnyBoy SB2500TLST-21 inverter queried from the Radzio! Modbus Master Simulator

  • SunnyBoy SB1.5-1VL-40 inverter queried from OPENHAB

Conclusion:

  • the behaviour of SunnyBoy SB2500TLST-21 inverter is causing the “Missing Byte” problem as already stated before by Rossko57
  • there is no defragmentation on the TCP/IP level

For details see test results below.

1. Testresult SunnyBoy SB2500TLST-21 inverter queried from OPENHAB

Response packet sent by SMA inverter in reply to the query.
Wireshark capture, packet length: 66 bytes

o	Network Interface:                          14 bytes
o	IP Header:	                                20 bytes
o	TCP Header:	                                20 bytes
o	Payload (MODBUS response message)           12 bytes
    	Transaction ID			2 bytes
    	Protocol ID				2 bytes
    	Length					2 bytes (Length=7)
    	Unit iD					1 byte
    	Function Code			1 byte
    	Byte Count				1 byte  (Byte Count = 4)
    	MSByte register 30775	1 byte
    	LSByte register 30775	1 byte
    	MSByte register 30776	1 byte  ( LSByte register 30776 missing !)

Wireshark reports:
Packet size limited during capture: Modbus truncated

The response packet is followed by a [PUSH] packet sent by the SMA inverter.
Wireshark capture, packet length: 60 bytes

o	Network interface:                            14 bytes
o	IP Header:                                    20 bytes
o	TCP Header:                                   20 bytes
o	Payload                                        6 bytes
    	LSByte register 30776		1 bytes
    	Padding bytes				5 bytes

The missing LSByte of register 30776 in the response message is not caused by defragmentation in the TCP/IP layers since:

  • The MF flag (More Fragments to come) in the IP Header of the response message is not set

  • The packet offset value in the IP Header of the [PUSH] packet is zero

It looks like OPENHAB is not processing the [PUSH] packet containing the LSByte of register 30776

Due to the lack of the LSByte the presentation of the actual current power is not a fluent line as shown below:

current power
current power1831×1006 457 KB

This missing LSByte is due to behavior of the SunnyBoy SB2500TLST-21.

I have no idea whether or not OPENHAB should be able to deal with this.

2.Testresult SunnyBoy SB2500TLST-21 inverter queried from the Radzio! Modbus Master Simulator

As with test 1, the LSByte of register 30776 is not contained in the response message but sent in a subsequent PUSH packet.

However , the Razio! Modbus Master Simulator includes the LSByte, received with the PUSH packet, in the result.

3. Testresult SunnyBoy SB1.5-1VL-40 inverter queried from OPENHAB

Response packet sent by SMA inverter in reply to the query:
Wireshark capture, packet length: 67 bytes

o	Network interface:                         14 bytes
o	IP Header:                                 20 bytes
o	TCP Header:                                20 bytes
o	Payload (MODBUS response message)          13 bytes
    	Transaction ID		   2 bytes
    	Protocol ID			   2 bytes
    	Length				   2 bytes (Length = 7)
    	Unit iD				   1 byte
    	Function Code		   1 byte
    	Byte Count			   1 byte  (Byte Count = 4)
    	MSByte register 30775  1 byte
    	LSByte register 30775  1 byte
    	MSByte register 30776  1 byte
    	LSByte register 30776  1 byte

All queried data is included in the response message.
The value are as expected , as is the presentation in OPENHAB.

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