Enriching the Industrial PLC Control Network
What would be the advantage of expanding your PLC system with intelligent remote I/O? The distributed intelligence would not only reduce the additional load on the PLC but also accommodate new functional requirements and special applications, allowing the system to grow while still under the same PLC's overall control. In essence, you could enrich your A-B system by expanding its size and functional capabilities, and still be standardized on Logix PLC systems.
Like your IT department, which may, for example, be standardized on Microsoft® software and Dell® computers but chooses printers or other peripherals from other vendors for specific purposes, you can continue to use Allen-Bradley software, PLCs, and training and maintenance contracts while choosing another vendor for specific I/O needs. Supplementing your industrial PLC system with intelligent remote I/O gives you options that may work well for your application. And in times of tighter budgets, using intelligent remote I/O may cost less, too.
Opto 22, a company well known for manufacturing reliable, industrially hardened I/O, offers an intelligent remote I/O system called SNAP I/O™ that fully supports EtherNet/IP, the protocol used by many A-B PLCs.
Because SNAP I/O supports EtherNet/IP natively, engineers currently using Allen-Bradley PLCs can supplement their control networks and expand capabilities without concerns about communication and compatibility, without extra programming, and with little effect on PLC performance.
PLCs usually scan remote I/O directly through a bus-coupler, which simply provides communication, not local intelligence. As we've seen, adding more I/O normally requires PLC resources both for scanning and for myriad functions such as counting, latching, thermocouple linearization, ramping, PID loop control, and so on. The addition of one or more PID loops can significantly impact system performance.
With Opto 22's SNAP I/O, however, all these functions and many more are distributed to the local I/O processor, which runs them independently. This intelligent I/O processor is called a brain. Sitting on the I/O rack, the brain is much more than a bus-coupler, offering not just communication but extensive local functions as well—including functions such as PID loop control that are more easily handled on the brain than on the PLC, because of their different design. With these functions independently handled at the I/O level, I/O point count can be added without significant impact on the PLC's performance.
To get this kind of local intelligence, you would expect to have to buy a supplementary PLC or mini-PLC and then program it with its own programming software. Opto 22's SNAP I/O brain, however, does not require programming; all of its functions are built in.
The SNAP I/O brain executes all of the following operations at the I/O level without additional programming:
- Engineering unit conversion
- Thermocouple linearization
- Temperature conversion
- PID loop control (up to 96 loops)
- Serial device control (RS-232/485)
- High-speed counting (up to 20 kHz, depending on the module and brain)
- Quadrature counting
- Analog scaling
- Offset and gain (calibration)
- Analog ramping
- Output clamping
- Filter weight
- Minimum and maximum values
- Digital and analog totalizing
- Watchdog timeout
- Time-proportional output (TPO)
- Input latching
- Pulse generation and measurement
- Frequency and period measurement
A free I/O configuration utility is included with the brain. Once the new I/O is configured, the brain scans its local I/O, performs required functions, and waits for the PLC to pick up the data.
Communication and Compatibility
If communication with the PLC is lost, the brain continues to run PID loops and other functions locally, so a communication failure does not necessarily cause local processes to stop. If a process needs to be brought to a safe condition in the event of communication failure—for example, valves or pumps turned off—the watchdog timeout can be enabled to monitor communications, and the brain will set outputs to a predetermined level if a failure occurs.
Conformance-tested by ODVA's Test Service Providers®, Opto 22's SNAP I/O is completely compatible with Allen-Bradley PLCs using EtherNet/IP, including ControlLogix, CompactLogix, and MicroLogix 1100/1400.