How to Set Up Gas Chromatograph Serial Communications

Emerson's Shane Hale


Gas chromatographs (GCs) are used in the oil & gas industry where critical measurement of the components in gas are required. GCs are used in custody transfer applications to more complex hydrocarbon C9+ analysis with dew point calculations, trace contaminant monitoring, and product quality verification.

A GC consists of a probe, sample system, chromatographic oven and controller to perform the sampling and analysis. But how does the information from the GC get back to the control system?

In this hour and ten minute recorded webinar, Communicating With Your Gas Chromatograph Thru Modbus: RS-232, RS-485, Ethernet, Emerson’s Shane Hale explains the options which are best for your application by first understanding the requirements needed and identifying the analysis information you plan to collect.

Shane opens by looking at the communications options available: serial, Ethernet, Foundation fieldbus and HART communication. The webinar specifically focuses on the serial and Ethernet communications options using the Modbus protocol.

Modbus is a serial communications protocol for communicating between a host device (master) and one or more slave devices. The protocol dates back to 1979 in Modicon PLCs and is a de-facto standard for industrial device communications. Shane notes that it is an ideal protocol for GC communications because of its availability, configurability and reliability. Getting it up and running can be a challenge, but once it is it tends to operate reliably over time.

The RS-232 physical protocol is 3-wire point-to-point only—single host to single slave with wiring distances up to 15 meters (50 feet). RS-422 is 4-wire and adds multi-drop with a single master and multiple slaves. RS-485 is more commonly used at RS-422 because it is two-wire, also multi-drop and has the same distance capabilities—up to 1200 meters (4000 feet).

Ethernet is a 4-wire (or fiber optic using converters) to provide multiple masters/multiple slaves multi-drop with wired distances up to 100 meters (320 feet) or greater distances via fiber optics. Ethernet is quickly becoming the preferred communications method.

For RS-232, the transmit (Tx) and receive (Rx) wires need to be crossed. This is the most common wiring mistake made when these are not crossed. The 3rd wire connects the signal grounds. One other common problem is that shielded twisted pair cable is not use, which can result in unreliable communications caused by electromagnetic interference (EMI).

RS-422 is 4-wire with two Tx and two Rx wires crossed, communicating in full duplex mode. Shielded twisted pair cable is also critically important to use. Multiple slave devices are wired in parallel. For RS-485 two wires are crossed, and full duplex can be converted to half duplex to reduce the number of wires required by connecting the positive Rx and Tx terminals together and the negative Rx and Tx terminals together on the master side. Shielded twisted pair cable is also highly recommended. Long cable lengths can create reflection issues which can be solved using 120-ohm resisters across the two wire endpoints on both the master and furthest slave device. Ethernet cabling pervasive and well defined.

Beginning at the 21-minute mark, watch the video for more on software settings, communications registers, alarm mapping, and other settings to assure successful and reliable communications. And at 46:40, Shane gets into the specifics of the data available in the Rosemount 77XA Natural Gas Chromatograph.

Connect and interact with the gas chromatograph experts at the Oct 1-5 Emerson Exchange conference and/or in the Analytical group in the Emerson Exchange 365 community.

Posted Tuesday, August 28th, 2018 under Analyzers.