What You Need To Know About The

Datalink GW1000

General discussion of failure to meet specifications

The following link describes some significant disasters thaI have been caused by process control failure.


Process control modules are used to both sense certain conditions (like temperatures, pressures etc.) and to control or regulate process components in the system that change things like flow rates (valves), heating inputs (burners, boilers etc). and other parameters that effect the operation of the process and determine the quality (and safety) of the output from the process.

From a safety perspective, electronic signals received from sensor modules and electronic signals sent to control modules are equally important. Even if all of the sensor and control modules are working properly, if the electronic signals they send or receive are too weak to be interpreted properly, the result can be a process control failure. The consequences of a process control failure can be as dramatic as the Bohpal, Three Mile Island or Texas City Disasters.

The pathway that carries these electronic signals is called a network. If, for some reason, the component parts of the network itself are deficient such that the electronic signals are too weak to be interpreted properly, both sensor information and control instructions can be negatively impacted. Electronic signal quality can decline as the physical length of the network increases or because of interference (often called "noise") from sources external to the network itself. Network specifications that describe how a network should be expected to function will usually quote a maximum physical length and will note any features of the network that are used to counter "noise" in the system and / or to preserve signal quality over this maximum physical length.

When writing these network specifications, the network manufacturer's engineers make assumptions about the minimum strength of the electronic signals that are sent by sensor modules and the minimum strength of the electronic signals that can be received and interpreted properly by control modules.

In the case of the Allan Bradley (AB) Data Highway Plus (DH+) network, AB's network engineers assume that all of the sensor and control modules will be ones that are manufactured by AB and, therefore, the minimum signal strengths sent and received by these AB modules are well known and can be assumed when writing the network specifications.

If, however, some of the sensor or control modules that are connected to the AB DH+ network do not reliably send or receive the assumed minimum strength electronic signals, the network may not be able to function properly over its specified maximum physical length. In addition, if the electronic signal strengths of these sensor or control modules are weaker than the assumed minimums, the noise-countering features of the network may not be able to maintain the necessary (and safe) signal quality.

Electronic signal strength is measured in units of "peak to peak voltage (Vpp)"* at the "port" (physical connector by which electronic signals enter or leave a process control module) of a process control module under various load (resistance measured in Ohms) conditions.

The tests previously described show that the peak to peak voltage (Vpp) measured for the Datalink GW1000 modules purportedly manufactured by Datalink are significantly below those measured for equivalent AB or Equustek modules.

This lower peak to peak voltage increases the risk that the Data Higway Plus network may not perform as specified. That is, it may not be safe to try to operate the DH+ network at its specified maximum physical length. These lower Vpp voltages also increase the risk that any noise-countering features of the AB DH+ network may not perform as specified. This may also contribute to unsafe conditions for those who may be exposed to a process control failure.

Most of the failures would not be dramatic. Many of them would go unnoticed, or the resulting action would probably be blamed on something other than the network. No trace of the error will remain after it occurs, other than the resultant damage.

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