I continued the investigation of the “lost byte problem”.
My power plant has 2 inverters:
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- SB 1.5-1VL-40
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- SB2500TLST-21
The “lost byte problem” shows up with the SB2500TLST-21 and not with the SB 1.5-1VL-40.
Without going into to much details:
SB 1.5-1VL-40
If register 30577 is queried with length = 2, all 13 bytes are transmitted in one MODBUS/TCP RESPONSE frame
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- Transaction ID 2 bytes
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- Protocol ID 2 bytes
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- Length 2 bytes
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- Unit ID 1 byte
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- Function code 1 byte
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- Byte count 1 byte
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- Register 30775 2 bytes
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- Register 30776 2 bytes
SB2500TLST-21
If register 30577 is queried with length = 2, the first 12 bytes are transmitted in a TCP [ACK] frame
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- Transaction ID 2bytes
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- Protocol ID 2 bytes
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- Length 2 bytes
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- Unit ID 1 byte
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- Function code 1 byte
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- Byte count 1 byte
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- Register 30775 2 bytes
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- Register 30776 MSByte
And the remaining byte is transmitted in the MODBUS/TCP RESPONSE frame
- Register 30776 LSByte
SB2500TLST-21
If register 30577 is queried with length = 4, the first 16 bytes are transmitted in a TCP [ACK] frame:
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- Transaction ID 2bytes
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- Protocol ID 2 bytes
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- Length 2 bytes
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- Unit ID 1 byte
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- Function code 1 byte
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- Byte count 1 byte
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- Register 30775 2 bytes
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- Register 30776 2 bytes
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- Register 30777 2 bytes
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- Register 30778 MSByte
and the remaining 1byte is transmitted in the MODBUS/TCP RESPONSE frame:
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- Register 30778 LSByte
The result from register 30775 / 30776 is as expected when querying with length = 4.
There is, however, a serious downside of extending the query length.
Instead of register 30776, now register 30778 is affected.
Register 30777 / 30778 hold the Inverters current line 1 power.
This value inside OPENHAB may drop down, but will be restored during the next query of the current line 1 power, there will be just a glitch !
But what if it applies to a register that holds data used to determine whether your shutters should be open or closed ?
At a polling frequency of 1 minute ………. imagine for yourself 