Modbus has been around for several decades and is widely implemented.  There are several elements of starting up a system with Modbus serial networks that usually get repeated. These include address, baud rate and parity. One element which does not get too much mention is the Modbus polarization resistors which can play a major role.

What does the polarization resistor do?

The detection margin requirement across the B and A signals is usually only about 200mV. With polarization resistors, the actual margin is ‘widened’ specifically when the signal drivers are not ‘driving’ thus allowing for better noise tolerance as well. This is especially true with networks that have variable speed/frequency drives  with high frequency switching being a large source of noise. The resistors are known as pull-up and pull-down resistors as they tie the B signal up to the 5V rail and the  A signal down to 0V.

On a recent startup, the PLC and HMI program was downloaded and the control program was being tested when the system started to act erratically. Closer observations revealed that the Modbus serial communications with the drive was sporadically dropping out, specifically when the drive was running the motor. All read values were becoming zeroes when this happened.

After trying several things, the polarization resistors solved the problem.

Things to note about polarization resistors:

1. Applied only once on each network. PLC’s may have this as an optional item in the Modbus port configuration. Slave devices usually do not have it though many manufacturers will advise on how to apply if when needed.
2. The presence of polarization resistors could reduce the number of slaves that can be had on the network, specifically if a lower pull-up or pull-down resistance is used. This and many other important points about Modbus is covered in the specification at Modbus.org.
3. What size resistors to use? The Modbus standard specification calls for resistance between 450 ohms and 650 ohms. Manufacturers usually have a guide. A good example to calculate it out is noted on page 11 of this document.

Polarization resistors are one factor to note in troubleshooting Modbus serial networks, others include proper grounding practices and proper usage of terminating resistors.