I am trying to interface with a temperature controller using Modbus RTU. I am not very familar with the topic, but I have been able to write code that can read the temperature of my three temperature controllers, but when I try to write to the three temperature controllers in sequence I get an error. The code I have been running is :
import minimalmodbus
import time
import serial
from datetime import datetime
minimalmodbus.BAUDRATE= 9600
minimalmodbus.PARITY = serial.PARITY_EVEN
Left = minimalmodbus.Instrument('COM3', 1,)
Middle = minimalmodbus.Instrument('COM3', 2)
Right = minimalmodbus.Instrument('COM3', 3)
print(Left.read_register(4096, 1))
print(Middle.read_register(4096, 1))
print(Right.read_register(4096, 1))
newTemp = 65
Ltemp = newTemp
Left.write_register(4097, Ltemp, 1)
time.sleep(.04)
Mtemp = newTemp
Middle.write_register(4097, Mtemp, 1)
time.sleep(.04)
Rtemp = newTemp
Right.write_register(4097, Rtemp, 1)
Again when I print the current tempearture the code does not give an error, and if I write to only one of the registers at a time (comment out the others and run them) it changes the setpoint temperature and does not throw an error.
More interestingly to me, when I do run this code, the first time I run it the setpoint of the "Left" temperature controller is changed and then an error is thrown. The next time I run the code the setpoint of the "Middle" temperature controller is changed and then an error is thrown. The last time I run the code the set point of the right temperature controller is changed with no error thown.
I tend to get different errors and I cannot really disern a pattern. They include:
Checksum error in rtu mode: '\x10\x01' instead of '\x01ì' . The response
is: '\x01\x10\x10\x01' (plain response: '\x01\x10\x10\x01')
builtins.ValueError: Too short Modbus RTU response (minimum length 4
bytes). Response: '\x02'
builtins.OSError: No communication with the instrument (no answer)
I have tried changing the baud rate to 4800, nothing worked when I did this. I also got similar errors changing from Modbus RTU to ascii. I was also getting the same erros when the parity was set at the defualt none. I also tried to sleep the code and change the timeout, which I think helped with the last error.
Any suggestions of what to try to stop getting these errors would be much appreciated.
Thank you
Edit
I tried deleting the number of decimals but still get the same error, the set point now updates so that 80 turns into 8.0, not that that's a big deal.
this is the full response I get when I run the code, now without the number of decimals:
19.1
19.1
21.1
Traceback (most recent call last):
File "C:/Users/matth/Desktop/tempController.py", line 26, in <module>
Left.write_register(4097, Ltemp)
File "c:\Users\matth\AppData\Local\Programs\Python\Python36-32\Lib\site-packages\minimalmodbus.py", line 296, in write_register
self._genericCommand(functioncode, registeraddress, value, numberOfDecimals, signed=signed)
File "c:\Users\matth\AppData\Local\Programs\Python\Python36-32\Lib\site-packages\minimalmodbus.py", line 697, in _genericCommand
payloadFromSlave = self._performCommand(functioncode, payloadToSlave)
File "c:\Users\matth\AppData\Local\Programs\Python\Python36-32\Lib\site-packages\minimalmodbus.py", line 798, in _performCommand
payloadFromSlave = _extractPayload(response, self.address, self.mode, functioncode)
File "c:\Users\matth\AppData\Local\Programs\Python\Python36-32\Lib\site-packages\minimalmodbus.py", line 1032, in _extractPayload
response))
builtins.ValueError: Too short Modbus RTU response (minimum length 4 bytes). Response: '\x01'
Left's temperature does change though. I still also get the instrument timeout occasionally error, but the error above seems to be the most common one. I think I will check how I wired the interface though, as I am wondering if that is the issue.
Edit II
I think I figured it out, I just had to increase the timeout time (I thought I already had but I guess not enough) minimalmodbus.TIMEOUT = 3 seems to make everything work.
Thanks for the help!
