FAQ: MODBUS

FAQ – Technical Update for MODBUS on Alicat

Alicat devices can be ordered with either the serial Modbus RTU or the Modbus TCP/IP protocols. Some common information on these protocols is compiled here.

What devices is Modbus available on?

Modbus RTU is available on all Alicat products, including our OEM Basis and EPC devices.

Modbus TCP/IP is not available on:

  • BASIS and EPC OEM devices.
  • Battery powered devices.
  • Hazardous area certified devices (class 1 div 2)

What are the differences between Modbus RTU and Modbus TCP/IP?

Modbus RTU is a master/slave based serial protocol which uses standard serial communication interfaces such as RS-232 or RS-485.

Modbus TCP/IP uses the same command set as Modbus RTU. Instead of serial the TCP/IP protocol and off-the-shelf network hardware to carry messages instead of serial cables.

What are the advantages of choosing a Modbus protocol?

Modbus is a mature protocol with over 40 years of history, so it has broad support among industrial control hardware and software systems. There are several open source libraries, free SCADA platforms, and plug-ins for control platforms like LabVIEW which support Modbus allowing easy development and integration of Modbus into existing control systems.

Modbus includes several error checking methods such as CRCs to protect against data transmission errors, making it suitable for systems where high reliability is desired.

Modbus RTU communicates via standard serial interfaces and Modbus TCP/IP uses off-the-shelf network components. This means Modbus can be connected to existing networks and computers without the purchase of new or specialized hardware, unlike most other automation protocols. If you would like to control an Alicat device over a network from a standard PC then Modbus TCP/IP is the best option.

Why choose a different protocol?

The ASCII serial interface which comes by default on all Alicat devices is still the most powerful and feature rich way to control an Alicat device.

Analog signals are currently the fastest way to control an Alicat device: analog I/O is updated every millisecond, while serial and Ethernet based communications all introduce some additional delay.

More modern protocols such as DeviceNet™, EtherNet/IP™, or EtherCAT™ offer advanced timing and fault recovery features which are unavailable on Modbus.

What are the specification changes to an Alicat Instrument, when Modbus is included?

Modbus RTU operates in parallel to our standard serial interface, and does not change any specifications of any Alicat Modbus device.

Modbus TCP/IP is not available with analog signals.

Modbus TCP/IP requires adds an additional 120mA of power consumption to any device.

How fast are communications with an Alicat instrument equipped with Modbus

Modbus RTU’s speed is limited by the speed of the serial bus, ranging from 40hz at 19200 baud up to 200hz at 115200 baud.

Modbus TCP/IP operates at approximately 50hz.

Can I retrofit Modbus to my existing Alicat devices?

Newer Alicat devices with a serial communications interface can be updated to support Modbus-RTU. Alicat devices with an Ethernet adapter card such as existing EtherNet/IP or EtherCAT card can be reconfigured to support Modbus TCP/IP.

What control systems or software does Alicat recommend for controlling a Modbus device?

There are so many options for building Modbus controls that it can be challenging to make a single recommendation.

Modbus Poll is a good testing and simulation tool for early validation and learning the Modbus protocol.

If you have python experience then the python library PyModbus is well documented and flexible and a good solution for building custom and flexible Modbus systems.

If you are looking for dedicated hardware to control your application you have many options as virtually any PLC will support Modbus. Automation Direct has affordable PLC and HMI solutions, and Schneider electric could be considered the reference implementation for Modbus devices.

VTScada is a commercial solution which provides good tools for building control interfaces and GUIs. It offers a security layer for remote/internet based controls and an excellent free demo so you can try it before you commit to a full software suite.

Where can I find more info?

The Modbus operating bulletin can be found on our manual page. Our applications engineers are also here to help with any questions.  Start a chat in the lower right corner, or give us a call at 520-290-6060.

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