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RS485 Pinout when using PyModbus
Apr 21, 2017 at 9:41am
I am trying to control a RS-485 RTU device (Love 16C-3) using a Python script (pymodbus library) that runs on a Windows 10 PC. The hardware is connected to the computer using a USB-RS232(http://www.kynix.com/Parts/3903714/RS232-GM-355.html) cable.
In my initial test, the Python script is not able to read anything off the RS-485 device.
What is the RS-485 pin out for the DB9 connector?
Should a USB-RS485 cable be used instead?
In my initial test, the Python script is not able to read anything off the RS-485 device.
What is the RS-485 pin out for the DB9 connector?
Should a USB-RS485 cable be used instead?
Apr 22, 2017 at 9:55am
RS-232 and RS-485 are two different specification's.
RS-232 is 3 wires full duplex hardware, with 1 receive and 1 transmit wire, and one logical ground. The signal levels are 3V to 15V for a low bit and -3V to -15V for a high bit. Any voltage levels outside of these is not valid.
RS-485 is 2 wires half duplex hardware, receive and transmit both use the pair of wires. Wire A is the normal signal and wire B is the inverse signal. RS-485 does no have defined signal voltages as it uses the differential between A and B to determine the bit. There must be at least 200mV differential between the two wires to count as a valid bit.
You probably need to buy a USB-RS485 virtual com port, they're not too expensive. Personally I recommend FTDI, we use them a lot at work and haven't had much issue with FTDI but we have with other companies chips.
Once you've got that you need to get your serial settings correct and try talking to your device.
Best of luck :)
RS-232 is 3 wires full duplex hardware, with 1 receive and 1 transmit wire, and one logical ground. The signal levels are 3V to 15V for a low bit and -3V to -15V for a high bit. Any voltage levels outside of these is not valid.
RS-485 is 2 wires half duplex hardware, receive and transmit both use the pair of wires. Wire A is the normal signal and wire B is the inverse signal. RS-485 does no have defined signal voltages as it uses the differential between A and B to determine the bit. There must be at least 200mV differential between the two wires to count as a valid bit.
You probably need to buy a USB-RS485 virtual com port, they're not too expensive. Personally I recommend FTDI, we use them a lot at work and haven't had much issue with FTDI but we have with other companies chips.
Once you've got that you need to get your serial settings correct and try talking to your device.
Best of luck :)
Last edited on Apr 25, 2017 at 1:27pm
Apr 23, 2017 at 10:15amclosed account (1vRz3TCk)
| RS-232 is 2 wires full duplex hardware... |
Apr 25, 2017 at 1:27pm| CodeMonkey wrote: |
|---|
| RS-232 has a minimum of three wires for full duplex. |
Quite right... I've updated my post to include a logical ground too.
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