IO module replacement: What you need to know
If your facility needs the features and added functionality of that new processor family, first determine if you need to replace the I/O modules. Many processor brands will communicate with legacy I/O systems and still allow most if not all the functionality of more modern control platforms. This may not be the case if changing processor brands.
Assuming it is time to replace the I/O system, there are several factors you need to consider when planning this activity. First and foremost is facility operation while this upgrade is occurring. What types of downtime windows are available for the cut-over? Does part of the old I/O system need to stay operational while the new system is being installed? Can the field wiring remain intact, or does that need replacement as well? Are you reprogramming the new processor as part of this upgrade?
How much I/O at a time
The downtime window will drive how much I/O can be replaced at one time. It will always be easiest to replace complete I/O enclosures at one time, if enough time exists. There are a couple of things that can be done to reduce the amount of cutover time for the new I/O system.
If the field wiring remains intact, new panel sub-plates can be pre-assembled and prewired to new terminal strips. When the downtime window occurs, disconnect the field side of the terminals on the old sub-plate; completely remove the old sub-plate; install the new sub-plate; and reconnect the field wiring sides of the new sub-plates. This approach may require installing new sub-plate mounting studs in the existing enclosure.
If changing sub-plates is not an option, ensure the new I/O platform physically fits on the old sub-plate (this is usually the case as I/O systems tend to get physically smaller over time), then remove, mount, and rewire the new I/O system on the existing sub-plate. In this case, keep the field side of the terminal strip intact. To save time you could consider prewiring and tagging the I/O module swing arm in advance (if your I/O module supports this).
I/O replacement time savings
When field wiring needs to be replaced, there are more challenges but also some opportunities. Wire can be prepulled, tagged, and spooled on both ends (field device side and new panel side) allowing for a faster cut-in.
Having portions of an existing I/O system remain intact while newer portions come online is the most challenging. This will require new wiring in most cases and parallel operations of old and new systems. For daisy chained devices using a common power supply, try to keep the entire loop intact on one system or the other if at all possible; this will simplify your life.
Where individual instrumentation input signals need to be shared between both systems, there are generally a couple of options available. Having signals networked between the old and new systems will be the simplest solution from a wiring standpoint, and there will be no loss of signal fidelity, but specialty communication modules may be required. For certain critical or high-speed parameters, this approach may not suffice. In that case you will need to pick one system to control and measure, and have the other request and/or monitor these results.
Dual system control
For output devices that need to be controlled from both systems, there are a couple of options to consider, depending on the device type.
Networking between the old and new systems is one common option. One system would physically control the device; the other would request activation or deactivation of the device. This is the most secure method of operation and the easiest to troubleshoot should problems occur during the transition process. This works well for both digital and analog devices.
If networking is not an option, interposing relays with dual contacts can be used to allow both systems to simultaneously control one digital output device, but care must be taken with this approach. If either system requests the device, it will unconditionally activate unless otherwise interlocked. If wired and designed properly, when the cutover is complete, the old system can be disconnected from the relay, the relay coil can be removed (a point of failure), and the relay base simply exists as a terminal block between the I/O module and field device in the new system.
Analog output devices are more problematic and not as easily adapted to this approach due to potential loss of signal fidelity. If you simply must control an analog device from two control systems and the systems cannot be networked, if all else fails, place the control (and associated measurement if necessary) in one system, and hardwire the setpoint or output signal (and any associated permissives) between them. Old-school techniques would typically do this with binary coded decimal (BCD) modules on both ends, although other approaches also are possible.
For pneumatically actuated digital devices, pneumatic switching may be used, although this is a less common option.
While there can be many challenges to an I/O upgrade, they are inevitable and worth the trouble, especially with older systems. With proper planning and attention to detail, these upgrades can go smoothly with minimal interruption to your facility.
David McCarthy is president and CEO of TriCore Inc., a Racine, Wisc.-based system integrator.