Gefran 800P and minimalmodbus (Mac): "ValueError: Too short Modbus RTU response (minimum length 4 bytes). Response: '\x00' "

Question

I am a beginner in Python (FYI) and trying to communicate with the Gefran 800P programmer/controller using minimalmodbus library (Python + Mac). I am using USB to RS485 Serial Converter Cable (http://www.ftdichip.com/Support/Documents/DataSheets/Cables/DS_USB_RS485_CABLES.pdf) to communicate with the Gefran device. Pins from the rear of the device (15+17(Rx+Tx) & 16+18(Gnd+Gnd)) are connected to Data +(A) and Data -(B) of the converter cable, respectively (RS485-2wired connection). I don't know the slave address for this device and there is no where mentioned about the address in the manual (http://www.primusthai.com/marketing/Gefran%20800%20(Controller).pdf).

I have tried:

1. Changing address of the slave form 0 to 247 and I always get the same message.
2. Tried checking for different register address and played around with function codes (3&4), played around with the programmer settings making sure both parameters in the code and the device are the same. Nothing works! Maybe I am missing something. Looking forward for your help. I am really stuck at this point for more than a week now!

Below is my code and when I run it, it gives me an error "ValueError: Too short Modbus RTU response (minimum length 4 bytes). Response: '\x00' ".

import minimalmodbus
import serial
import sys
import time
import sh

minimalmodbus.CLOSE_PORT_AFTER_EACH_CALL = False 
print "scanning for addresses..."
for i in range(1,247):


    programmer = minimalmodbus.Instrument('/dev/tty.usbserial-FT0C9E5S', i, 'rtu') 
    print "address is", i
    programmer.mode = minimalmodbus.MODE_RTU
    programmer.serial.timeout = 8
    programmer.handle_local_echo = False
    # If RS-485 adapter has local echo enabled  
    programmer.serial.bytesize = 8
    programmer.precalculate_read_size = True  
    programmer.serial.baudrate = 9600
    programmer.serial.parity = minimalmodbus.serial.PARITY_NONE
    programmer.debug = True
    print "all parameters set... now reading"
    print programmer 


    try:

       setpoint = programmer.read_register(79,1,4)
       print setpoint
       print "This is the address", i
    except sh.ErrorReturnCode_1:
        print "no response"

Answer 1

Okay Jacek, thanks for all the hints.. I finally managed to get the device communicating using Gefran Cable. I am not 100% sure what really worked, but I tried the following:

1. Install software "modpoll" to poll for the modbus devices to compare the parameters in the GF_eXpress software and the device and to know what is happening in the background.

2. Tried to use pyserial to serially communicate with the device - I think this did the trick, you have to first initialise the port and then communicate. In my case opening a serial port and connecting to the port and then using modbus, worked like magic.

3. I restarted the programmer, my Mac, gave basic parameters baudrate - 9600, parity - none, address - 1 and stop bits - 1 and it started working..

I am guessing all of these could have helped plus if nothing works, it is always good to restart every single device that you are using and reset the parameters and set it again to basic/standard values.

Answer 2

Pins from the rear of the device (15+17(Rx+Tx) & 16+18(Gnd+Gnd)) are connected to Data +(A) and Data -(B) of the converter cable

Maybe I'm wrong, but IMHO, you should use pins 16 & 15 for + and -

Pins 18 and 17 are described as:

Configurable serial line isolated to 1500V Passive current loop (max. 1200 baud)

maybe you shouldn't wire them together with 15 & 16?