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SDM XXX meters
These meters are sold from Eastron or Denson.
The SDM-120 defaults to 2400 baud, 8 data bits, 1 stop bit, no parity. To change
any of these settings, you must press and hold the single button on the face of
the meter for about 3 seconds, until the display shows - SET -. Then you can
update Modbus registers using Modbus function code 0x10, always writing 2
registers because the values are stored as 32-bit IEEE-754 floating point
values.
For example, to change the baud to 9600 (the fastest it supports), you need to
set address 28 to 2, which in raw Modbus translates to setting 2
registers via function code 0x10 (decimal 16) at address 0x1C to 0x4000 0x0000 (0x40000000 is the IEEE-754 representation for 2.0). The raw Modbus
message and response looks like this:
-> [01h] [10h] [00h] [1Ch] [00h] [02h] [04h] [40h] [00h] [00h] [00h] [E7h] [36h]
<- [01h] [10h] [00h] [1Ch] [00h] [02h] [80h] [0Eh]
Once changed, the meter must be reset by removing and re-applying power.