September 2013 Newsletter

Modbus TCP

For many reasons factory floors have become a mishmash of automation devices. There are PLCs, VFDs, servo controllers, HMIs, and intelligent sensors from different suppliers on the same floor.

Communication among these devices has been accomplished using proprietary networks when the devices are from the same manufacturer or using discrete/analog I/O when the devices are from different manufacturers. A viable alternative has existed for many years – Modbus TCP. The Modbus protocol was developed in 1979 by PLC manufacturer Modicon which is now part of Schneider Electric.

Originally a serial communication scheme it now utilizes standard Ethernet. In recent years, Schneider Electric transferred control of the protocol to the Mod- bus Organization (an independent group of users and suppliers that seeks to promote its use). The Modbus Organization describes the protocol as follows. ‘It is used to establish master-slave/client-server comunication between intelligent devices. It is a de facto standard, truly open and the most widely used network protocol in the industrial manufacturing environment.

It has been implemented by hundreds of vendors on thousands of different devices to transfer discrete/ana- log I/O and register data between control devices. It’s a lingua franca or common denominator between different manufacturers.’ There are an estimated 7 million nodes in Europe and North America alone. A brief discussion of three examples follows.

First, a PLC is used to change the speed set point of a VFD. More traditional methods may have included using an analog output
card to change the 0 – 10 VDC speed reference on the drive or discrete outputs to change a configured speed preset. A single Ethernet connection between the PLC and VFD using Modbus TCP not only allows for on-the-fly changing of the speed reference but also for chang ing parameters such as ‘accel’ and ‘decel’ and for reading drive values such as actual motor amperage. Second, an operator is required to change servo parameters such as move distance, accel and speed depending on the product being produced. This could be done using an HMI and PLC with a bundle of discrete I/O for a finite combination of moves (usually 8 – 16). Or it can be accomplished using Modbus TCP with just an HMI connected to the servo control-ler. With Modbus TCP, parameter move combinations are infinite and status values can be displayed on the HMI. Also, the home offset can be changed without using a PC and servo drive interface software. Final- ly, the exchange of register data between PLCs of different manufacturers can easily be accomplished with Modbus TCP. Traditionally, this may have required SCADA software connected to both PLCs or through discrete I/O using 10-bit BCvalues. Visit Modbus.org for more information.

Hiring Outside Technical Resources (continued from August’s newsletter)

A reader response from last month requested suggestions on how to identify good resources. Here are some ideas. Hire them like you would a permanent employee – consider giving them a test during an interview!

Have a software engineer write code to solve a problem.

Or have a technical writer produce a short manual on how to change a car tire. Another idea is to hire the resource for a simple project with a defined set of expectations, due-dates and rules to follow. If they do well, make the next project more involved. Finally, basic networking can help locate good resources. Attend trade organization meetings and start asking questions…..