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Physical layer diagnostics: Part II

Jan. 5, 2007
In this month’s installment of On the Bus, process control specialist John Rezabek summarizes the current and soon-to-be-released offerings of four of the leading suppliers of physical-layer hardware.
By John Rezabek, Contributing Editor

Last month’s column discussed the need for physical layer diagnostics, specifically some form of 24/7 vigilance for the cable, power conditioners, terminations and other “physical” aspects of one’s fieldbus networks. One applies these to selected segments, depending on the perceived risks and consequences in specific applications. Diagnostics for a refinery or a drilling rig in Siberia may be more extensive than those in a compact, indoor pharma plant. A few products are already available, and more will be released soon.

The first to market in North America is Pepperl + Fuchs’ ADM (advanced diagnostic module) in both a mobile and a fixed version that mounts to P + F power hubs. The fixed version monitors up to four segments continuously, while the mobile handles one segment at a time.

The most detailed information about the segment(s) is revealed using field device tool/device type manager (FDT/DTM) software. Oscilloscope traces can be captured and examined in detail using the DTM. Fieldbus nerds with faster-spinning beanie-copters than mine can even filter transactions by type! This feature can be very useful if your device and DCS suppliers are debating who has an implementation problem.

The software automatically detects excessive noise, missing terminators, jitter (variation/drift in the zero-crossing timing of the waveform) and current/voltage problems. It records a “baseline” and automatically sets alarm levels for all the variables it monitors, including the voltage and signal strength at each device.

MTL-Relcom is close to releasing a diagnostic module for its redundant eight-segment F890/892 power conditioners. The module appears as an H1 device on the first of the up-to-eight segments powered from the 890/892 power backplane. Peak and average noise levels in up to three frequency bands and signal levels at up to 32 fieldbus devices are updated in the device’s transducer block, along with the DC voltage at each segment. g interfaces for their systems.

Like the P + F, the MTL-Relcom device also has a simple contact closure that can be wired as a “common trouble” discrete input to the host.  While the MTL-Relcom solution consumes a “device” slot on one segment out of eight and adds a degree of asynchronous communications load, I don’t see this as a huge issue for most of us. Some argue that sending diagnostics on the same network that’s having problems is a mistake. I’d counter that, if your network is so bad that not even the diagnostics are getting through, you will know it and have bigger problems to solve.

MTL also plans to offer a one-per-segment solution that can be attached to any network, regardless of whose power conditioner is used.

Turck demonstrated a diagnostic module for up to 16 segments, powered from up to four of its soon-to-be-released four-segment, redundant power conditioner backplanes at the ISA Expo. It gathers and sends diagnostic information independent of the H1 segment or the DCS H1 interface, and uses high-speed Ethernet (HSE) to interconnect the modules and power backplanes. A Windows PC running DTM software is used for viewing the individual segment data, which includes device signal levels and jitter. Turck sets default alarm levels and also provides a contact closure for sending a “common trouble” alarm to the DCS. The new redundant power conditioner backplanes and HSE diagnostic modules are expected to ship in mid-2007.

MooreHawke has already released a diagnostic module that monitors power supply voltages from its conditioner backplane and includes fixed alarms for noise on each segment, alarming when noise exceeds 75 mV.

Perhaps by mid-2007, its advanced diagnostic module will be available. MooreHawke’s plan is for diagnostics to interface with the host over Modbus (serial or TCP/IP), rather than consume H1 resources. I get the impression it’s intended for a sampling of segments only—those selected by the end user as suspect or “problems.”

Chances are, if you’re planning a new project, you’ll have some leverage to bundle the advanced diagnostics capabilities with your power conditioner, field termination and/or DCS hardware purchase. If you do, I think it will pay out, not only during installation and commissioning, but also a few years down the road, when your local climate starts encroaching on your once-pristine installation.

  About the Author
John Rezabekis a process control specialist for ISP Corp., Lima, Ohio. Email him at[email protected].

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