Hi. I use your post to connect my sdm120 - it works, thank you 
I never deal with modbus, can you share your modbus.cfg with other parameters for sdm120 like apparent power, reactive power, power factor, etc?
Here it is:
# read voltage registers
modbus:serial.sdm120V1.connection=/dev/ttyUSB0:2400:8:none:1:rtu
modbus:serial.sdm120V1.type=input
modbus:serial.sdm120V1.valuetype=float32
modbus:serial.sdm120V1.length=2
modbus:serial.sdm120V1.start=0
modbus:serial.sdm120V1.id=1
# read current registers
modbus:serial.sdm120V2.connection=/dev/ttyUSB0:2400:8:none:1:rtu
modbus:serial.sdm120V2.type=input
modbus:serial.sdm120V2.valuetype=float32
modbus:serial.sdm120V2.length=2
modbus:serial.sdm120V2.start=6
modbus:serial.sdm120V2.id=1
# read power registers
modbus:serial.sdm120V3.connection=/dev/ttyUSB0:2400:8:none:1:rtu
modbus:serial.sdm120V3.type=input
modbus:serial.sdm120V3.valuetype=float32
modbus:serial.sdm120V3.length=2
modbus:serial.sdm120V3.start=12
modbus:serial.sdm120V3.id=1
# read Apparent power registers
modbus:serial.sdm120V4.connection=/dev/ttyUSB0:2400:8:none:1:rtu
modbus:serial.sdm120V4.type=input
modbus:serial.sdm120V4.valuetype=float32
modbus:serial.sdm120V4.length=2
modbus:serial.sdm120V4.start=18
modbus:serial.sdm120V4.id=1
# read Reactive power registers
modbus:serial.sdm120V5.connection=/dev/ttyUSB0:2400:8:none:1:rtu
modbus:serial.sdm120V5.type=input
modbus:serial.sdm120V5.valuetype=float32
modbus:serial.sdm120V5.length=2
modbus:serial.sdm120V5.start=24
modbus:serial.sdm120V5.id=1
# read Power factor registers
modbus:serial.sdm120V6.connection=/dev/ttyUSB0:2400:8:none:1:rtu
modbus:serial.sdm120V6.type=input
modbus:serial.sdm120V6.valuetype=float32
modbus:serial.sdm120V6.length=2
modbus:serial.sdm120V6.start=30
modbus:serial.sdm120V6.id=1
# read Power factor registers
modbus:serial.sdm120V7.connection=/dev/ttyUSB0:2400:8:none:1:rtu
modbus:serial.sdm120V7.type=input
modbus:serial.sdm120V7.valuetype=float32
modbus:serial.sdm120V7.length=2
modbus:serial.sdm120V7.start=70
modbus:serial.sdm120V7.id=1
# read Import active energy registers
modbus:serial.sdm120V8.connection=/dev/ttyUSB0:2400:8:none:1:rtu
modbus:serial.sdm120V8.type=input
modbus:serial.sdm120V8.valuetype=float32
modbus:serial.sdm120V8.length=2
modbus:serial.sdm120V8.start=72
modbus:serial.sdm120V8.id=1
# read Export active energy registers
modbus:serial.sdm120V9.connection=/dev/ttyUSB0:2400:8:none:1:rtu
modbus:serial.sdm120V9.type=input
modbus:serial.sdm120V9.valuetype=float32
modbus:serial.sdm120V9.length=2
modbus:serial.sdm120V9.start=74
modbus:serial.sdm120V9.id=1
# read Import reactive energy registers
modbus:serial.sdm120V10.connection=/dev/ttyUSB0:2400:8:none:1:rtu
modbus:serial.sdm120V10.type=input
modbus:serial.sdm120V10.valuetype=float32
modbus:serial.sdm120V10.length=2
modbus:serial.sdm120V10.start=76
modbus:serial.sdm120V10.id=1
# read Export reactive energy registers
modbus:serial.sdm120V11.connection=/dev/ttyUSB0:2400:8:none:1:rtu
modbus:serial.sdm120V11.type=input
modbus:serial.sdm120V11.valuetype=float32
modbus:serial.sdm120V11.length=2
modbus:serial.sdm120V11.start=78
modbus:serial.sdm120V11.id=1
# read Total active energy registers
modbus:serial.sdm120V12.connection=/dev/ttyUSB0:2400:8:none:1:rtu
modbus:serial.sdm120V12.type=input
modbus:serial.sdm120V12.valuetype=float32
modbus:serial.sdm120V12.length=2
modbus:serial.sdm120V12.start=342
modbus:serial.sdm120V12.id=1
# read Total reactive energy registers
modbus:serial.sdm120V13.connection=/dev/ttyUSB0:2400:8:none:1:rtu
modbus:serial.sdm120V13.type=input
modbus:serial.sdm120V13.valuetype=float32
modbus:serial.sdm120V13.length=2
modbus:serial.sdm120V13.start=344
modbus:serial.sdm120V13.id=1