Fiber Modems with Modicon Remote I/O HSBY systems

By Bob Culley

Delta Automation was tasked with re-configuring and certifying a Modicon remote I/O system during a recent plant-wide outage at a huge industrial chemical plant. This system consisted of a Quantum hot standby (HSBY) system with dual remote I/O cables. One drop was located a fairly long distance away in an adjacent building and was linked via fiber optic modems. This system has never performed as specified. The hot standby system had never switched properly and the remote I/O system was prone to errors and dropouts. Several years ago, the client had the manufacturer send in a service technician to repair the system. The technician immediately stated that the RG-6 cables were too close to the 110 VAC wiring and would not work. This is a typical “knee-jerk” reaction to a system issue. Using modern Quad-shield cable with the proper grounding, 110 VAC is almost never an issue with Modicon remote I/O. Close proximity to 480 VAC is another story.
I visited the site for a system “re-con” several months ago. Within a very few minutes, I picked out several real issues. As I investigated further, there were quite a few things that needed attention.
Some of these things were:
No hot standby in-line terminators installed
The drop hold up time was left at the default value (see our Summer 2003 newsletter)
The trunk cable was improperly terminated at the fiber modem
The fiber optic modems were improperly terminated
Several RG-6 F connectors were improperly installed and are a problematic type (see our Fall 2002 newsletter)
The manufacturers’ field service technician failed to mention these issues.

This system is vital to the entire plants operation, and only gets a planned outage about every eighteen months or so. It was planned to perform the repairs to these issues during the next planned shutdown. During the discussion of this plan it was decided to replace all of the cables, taps, splitters, etc. for the entire system, since it rarely stops production. All of the parts were assembled together and pre-positioned at the system for the planned outage. When the appropriate time arrived, the system was stopped and the entire remote I/O system was stripped out. For the plant personnel that had been warned, “Not to touch” any of the cables, it was a shock to see it being torn apart.
The re-assembly plan was to install the new trunk, HSBY and drop cables plus install all new splitters and taps with insulating boots on the connection points to reduce any contaminants from entering the connections. Also the fiber optic modem, a third party unit, was to be moved to a position in the system which would allow more signal strength to the unit. Where it was currently located, the signal loss was about –16.00 dB, but the system was terminated directly into the fiber optic unit. This is an incorrect termination method for a Modicon remote I/O system. This fiber optic unit is specified to have a sensitivity level of –20.00 dB. Its’ new location had a signal level of –19.50 dB. This signal level was an actual measurement performed upon this system. That level is achieved by the sum of the splitter loss of –6.0 dB (used as a combiner in a HSBY system) plus the loss through a tap of –13.50 dB. However; once powered up, the fiber optic unit only performed intermittently. It was discovered that the unit was not getting a sufficient level of signal to operate. Luckily the plant had two spare Modicon fiber optic repeaters, which meet the Modicon specification of –35.00 dB, and they were installed to replace the fiber units on the head-end side of the fiber link. The secondary side of the fiber link was left as is. A little known fact is that these third party fiber units will communicate with the Modicon fiber optic units via the fiber.
Once this replacement was performed, the system testing/certification was completed. Everything was well within the factory specifications. The hot stand-by system was completely function tested and for the first time ever, was operational.
The entire system was rebuilt, tested and certified within the estimated time allotted.
A blanket statement was made that these third party fiber optic modems do not work with the Modicon HSBY systems.  Although partially correct, it is due to a signal level issue more so than an HSBY issue. However; having stated that, all Modicon HSBY remote I/O systems have a signal level at the first available drop, of between –19.50 dB and –20.00 dB, which is at the very limit of these third party fiber optic units. The sensitivity of the Modicon fiber optic unit is the entire specification range of –14.00 dB to –35.00 dB; therefore it will function at any properly operating location/drop on a remote I/O system.

UPDATE: October 6, 2010, the plant engineer for this system reports that zero errors have occurred on this remote I/O network since the completion of the certification.

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About deltaautomation

President of Delta Automation, Inc. Working in industrial automation in excess of 30 years
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