Modbus FAQ

Alicat devices are compatible with a variety of analog, serial, or industrial communication protocols for easy data collection and analysis. Options include analog (4-20 mA, 0-10 Vdc, 0-5 Vdc, or 1-5 Vdc), ASCII and Modbus RTU over RS-232, ASCII and Modbus RTU over RS-485, Profibus, DeviceNet, EtherNet/IP, Modbus TCP/IP, and EtherCAT.

In this article, we provide answers to common questions about Modbus.

Modbus manual

Where can I find supporting download files?

Find supporting downloads such as MBP files here.

Which products can have Modbus installed?

Modbus RTU is available on all Alicat products.
Modbus TCP/IP available on most product, with the exception of:
- OEM devices (BC, EPC, and EPCD-Series)
- Portable instruments (MB, PB, and LB-Series)
- Class 1 Division 2 area instruments (-X part numbers)

Communications protocols comparison

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

Modbus RTU is a master/slave based serial protocol using 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 cables, the TCP/IP protocol uses off-the-shelf network hardware to carry messages.

How does Modbus change instrument specifications?

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

Modbus TCP/IP has the following changes:

Alicat part number: -MODTCPIP
Analog signals?: No
Electrical connections: Comms over dual RJ45
Additional power requirement: 120 mA
Software compatibility: LabVIEW; Modbus Poll
Max number of devices on network: No limit

Alicat connector pinout diagrams

Do I still have a choice of gases and gas mixes on Modbus instruments?

Yes. Gas instruments with EtherCAT have the full Gas Select™ list of gas calibrations and Composer™ gas mixing firmware

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 40 Hz at 19200 baud up to 200 Hz at 115200 baud.
Modbus TCP/IP operates at approximately 50 Hz.

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 with 9v or earlier firmware can be reconfigured to support Modbus TCP/IP.

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

There are many options for building Modbus controls, making it 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, PyModbus is well documented and flexible solution for building custom 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.