Modbus-RTU: Only first byte of answer is read from my device

[gonium]

I'm implementing a client for a Eastron SDM630 electricity meter using your library. The aduRequest ist composed and send correctly, but your library only reads one byte back. I suspect that this relates to the handling of the select call in serial_linux.go, but I'm not sure.

My setup consists of a RS485-USB-dongle that shows up as /dev/ttyUSB0 under my Ubuntu 14.04-box. I can successfully read the device using the minimalmodbus python library and the following script:

#!/usr/bin/env python
import minimalmodbus

instrument = minimalmodbus.Instrument('/dev/ttyUSB0', 1) # port name, slave address (in decimal)
instrument.serial.baudrate = 9600
instrument.serial.bytesize = 8
instrument.serial.parity = minimalmodbus.serial.PARITY_NONE
instrument.serial.stopbits = 1
instrument.serial.timeout = 1
instrument.debug = True
instrument.mode = minimalmodbus.MODE_RTU
print instrument

l1_voltage = instrument.read_float(0, functioncode=4,
    numberOfRegisters=2)
print "L1 voltage: ", l1_voltage

The output of this script looks like this:

$ python test.py
minimalmodbus.Instrument<id=0xb7322b2c, address=1, mode=rtu, close_port_after_each_call=False, precalculate_read_size=True, debug=True, serial=Serial<id=0xb7322b6c, open=True>(port='/dev/ttyUSB0', baudrate=9600, bytesize=8, parity='N', stopbits=1, timeout=1, xonxoff=False, rtscts=False, dsrdtr=False)>

MinimalModbus debug mode. Writing to instrument (expecting 9 bytes back): '\x01\x04\x00\x00\x00\x02q\xcb'
MinimalModbus debug mode. No sleep required before write. Time since previous read: 1429260650749.2 ms, minimum silent period: 4.01 ms.
MinimalModbus debug mode. Response from instrument: '\x01\x04\x04Cj\xcf\x17\xdb\xe2' (9 bytes), roundtrip time: 31.6 ms. Timeout setting: 1000.0 ms.

L1 voltage:  234.808944702

So, in order to query the SDM630, the library generates the following ADU request (in python syntax):
'\x01\x04\x00\x00\x00\x02q\xcb' and the device responds with '\x01\x04\x04Cj\xcf\x17\xdb\xe2'.

BUT: My program creates the correct ADU request []byte{0x1, 0x4, 0x0, 0x0, 0x0, 0x2, 0x71, 0xcb} (go syntax). It fails to read the answer correctly, it just receives the first byte of the answer: []byte{0x1}. The output of the program is here:

$ go run main.go  
Connecting to RTU via /dev/ttyUSB0
test: 2015/04/17 11:07:40 modbus: connecting '/dev/ttyUSB0'
test: 2015/04/17 11:07:40 modbus: sending []byte{0x1, 0x4, 0x0, 0x0, 0x0, 0x2, 0x71, 0xcb}
test: 2015/04/17 11:07:40 modbus: received []byte{0x1}
Failed to read from SDM630 device modbus: response length '1' does not meet minimum '4'
test: 2015/04/17 11:07:40 modbus: closing '/dev/ttyUSB0'

Any ideas?

Best regards,
-Mathias

[nqv]

I should provide expected length to read from serial port for each command.
What if you put time.Sleep(300 * time.Millisecond) before syscall.Select (serial_linux.go:123), will you have full response data?

[gonium]

@nqv: Thank you very much. If I insert a sleep time, I can correctly receive the data (and the data is also correctly decoded, see my last commit). I guess that you can determine the sleep time from the baudrate and the expected response length.

[nqv]

@gonium Thanks. I'll fix this issue as you have suggested.

[nqv]

@gonium Thanks for your pull request, I believe the issue has been fixed. I'd be grateful if you can test and let me know if it works with your device.

You also need to run go get -u github.com/goburrow/modbus to also download github.com/goburrow/serial as I move the serial part to another repository.

Thanks

[gonium]

@nqv I just switched from my temporary fork back to your master - now everything works as expected. Please note that I am currently using the SDM630 device in its factory settings: 9600 8N1. I didn't test higher baud rates.

Thank you very much for your help! Closing this for now.

[arnavbansal95]

@gonium How to connect Pi to TTL to 485 Converter?

[gonium]

Hi @arnavbansal95, I prefer using cheap (as in 3 USD) USB-to-RS485 converters. If you really need to connect directly to the extension header of your RPi you can have a look at the schematics of this Sparkfun RS485 shield.

[arnavbansal95]

@gonium Thanks ....I also got a Usb to 485 converter....but I don't know how to install its driver on raspian as I am new to linux....could you help me with that?

[NilanjPatel]

@gonium how you did sleep thing to to read energy meter??

[surbhigup]

@gonium i am also working on it but didn't get the result. Can you give me some help that how you send query?