Sierra Monitor Corporation TR-005 manual Knowing the protocol, When a specification states that

Knowing the protocol

When a specification states that

aproduct has an RS-485 inter- face, it only defines the serial elec- trical hardware connection and not the protocol. It is important for the safety professional to also know the protocol. While hun- dreds of serial protocols exist in the instrumentation world, the most common serial protocol is Modbus RTU.

Likewise, when a specification states that a product has an Ethernet port, it only defines the form of the hardware interface, commonly known as a Cat-5 cable connection. The most common Ethernet protocol in the industrial world is Modbus/TCP. The Modbus/ TCP de-facto standard is being rapidly chal- lenged by EtherNet/IP in the industrial world and standards such as BACnet/IP in the build- ing automation world.

The gas monitoring system must be able to hand off data and accept inputs in the protocol format demanded by the plant-wide control system. In Figure 2 the original single system concept has been expanded to indicate that multiple gas detection controllers might be connected via a serial interface, but the pri- mary Information layer now includes an Ethernet backbone. Now multiple PCs can have access to data from the gas monitoring sys- tem. Many other devices will also reside on the backbone, including process control or moni- toring equipment, building management de- vices and control room systems.

Critical to the ability to connect to the plant backbone is the Communications Bridge or Gateway. This device enables hardware plat- form and protocol conversion interoperability between dissimilar systems and the backbone. The safety professional can breathe a sigh of relief because his choice of monitoring system is no longer restricted to the backbone proto- col.

New features

Once the Ethernet interface is established, new features of monitoring systems become avail- able. Possibly the most useful is a Web server function. A Web server can be installed in the Communications Bridge/Gateway or in another Ethernet node so authorized personnel have desktop access to graphical representation of the gas monitoring system status. Graphical representation is no longer relegated to ex- pensive, license-based, control room packages. Other features include report by exception, re- dundancy functions and remote datalogging, all enabled by the Communications Bridge and accessible via the plant backbone.

No heavy lifting here, it is all done for you.

Gordon Arnold is president of Sierra Monitor, a lead- ing manufacturer of plant-wide gas risk monitoring systems. He has been closely involved in the gas de- tection industry for more than 25 years. Gordon can be contacted at garnold@sierramonitor.com or (408) 262-6611.

© 2004 by Sierra Monitor Corporation

Sierra Monitor Corporation 1991 Tarob Court Milpitas, CA 95035 USA (408) 262-6611 (800) 727-4377 Fax: (408) 262-9042

Visit our Web site: www.sierramonitor.com E-Mail: sierra@sierramonitor.com