hopping someone here can help please. Im communicating with a modbus device and getting data back as expected, im pretty new to this so i need a bit of advice, im trying to read all of the bit registers on my device (Eaton SC200) for a custom HMI. If i try a for next loop or even as in the code posted, the com port looses communications.
#!/usr/bin/env python
import minimalmodbus
import time
instrument = minimalmodbus.Instrument('COM11', 1)
minimalmodbus.TIMEOUT = 1
DO =[]
instrument.address # this is the slave address number
instrument.mode = minimalmodbus.MODE_RTU # rtu or ascii mode
#port name, slave address (in decimal)#0 = instrument.read_bit(1,1)
#digital = instrument.read_registers(1,32,4)
Version =instrument.read_registers(3001,3,4)
serialupper = instrument.read_registers(6001,1,4)
serialLower = instrument.read_registers(6002,1,4)
busvolt= instrument.read_registers(7001,4,4)
print type(serialupper[0])
print(serialupper, serialLower)
upper = serialupper[0] << 16 # shift upper left 16 bits
Snumber = upper + serialLower[0]
print ('Serial no: = ' + str(Snumber))
print(busvolt)
print ('Getting i-o')
print instrument.read_bit(1001,2)
print instrument.read_bit(1002,2)
print instrument.read_bit(1003,2)
print instrument.read_bit(1004,2)
print instrument.read_bit(1101,2)
print instrument.read_bit(1102,2)
print instrument.read_bit(1201,2)
print instrument.read_bit(1202,2)
print instrument.read_bit(1203,2)
print instrument.read_bit(1204,2)
print instrument.read_bit(1201,2)
print instrument.read_bit(1201,2)
The response i get is this:
<type 'int'>
([3754], [53255])
Serial no: = 246075399
[16989, 42144, 32704, 0]
Getting i-o
0
0
Traceback (most recent call last):
File "modbusRTU.py", line 29, in <module>
print instrument.read_bit(1003,2)
File "C:\Python27\lib\site-packages\minimalmodbus.py", line 193, in read_bit
return self._genericCommand(functioncode, registeraddress)
File "C:\Python27\lib\site-packages\minimalmodbus.py", line 697, in _genericCommand
payloadFromSlave = self._performCommand(functioncode, payloadToSlave)
File "C:\Python27\lib\site-packages\minimalmodbus.py", line 795, in _performCommand
response = self._communicate(request, number_of_bytes_to_read)
File "C:\Python27\lib\site-packages\minimalmodbus.py", line 930, in _communicate
raise IOError('No communication with the instrument (no answer)')
IOError: No communication with the instrument (no answer)
I have changed the Timeout to various values from 0.5 to 2 to see if that is the issue, sometimes i get a full data run other times i get nothing. Any assistance would be gratefully received.
It's most likely a physical comms issue. Check your cable terminations, check the terminating resistors (for RS-485), check for RFI/EMI interference sources, and try lowering the baud rate. The longer the cable, the lower the baud rate you should use.
If this is RS-485, are the terminating resistors on both ends of the bus? And are they the correct size? This is more important on high speed networks. On slower (9600 and less) networks, I've seen RS-485 work fine with either no terminating resistors or with oversized terminators. Typically I'll set the RS-485 baud rate to 9600 if I'm not polling much data.