The developer creates a simulation logic fragment for each PLC discrete output. In its most basic form, PLPS is all discrete (digital). combinational and supervisory logic that monitors multiple sensors and drives device logic in response to stimuli.proportional, integral, derivative ratio and other higher-level control algorithms.sequence logic that energizes/deenergizes and otherwise communicates with the device logic fragments to operate the field equipment sequentially.Some examples of process control logic are:
Process control logic is logic that is not specific to a device, but that controls the on/off, start/stop, speed, percent open/closed, and other values that drive one or more sections of device logic. Other forms of device logic include (but are not limited to): the restart inhibit timer that prevents multiple restarts and gives the windings time to cool.the timer that monitors the motor's auxiliary contacts to ensure the motor starts within a reasonable time limit.the permissive logic that, when in auto mode, will allow the motor to start/stop based on a set of conditions.the interlock logic that will automatically shut off the motor in emergencies.Device control logic supervises a specific field device, like a motor, and can have the following attributes: Most programs resolve to two major sections: device control and process control. The primary goals are to simulate on/off for motors and open/close for valves, and cycle through sequences, demonstrate safety shutdowns, and then be able to demonstrate that the program reacts as it should to numerous upset scenarios. All it requires is a basic understanding of the external equipment and of how that equipment interacts with the PLC.įor most process applications, there is minimal need to simulate advanced control schemes.
Loop back device software#
It does not require any extra equipment or software packages.
Loop back device Pc#
While ideally the PLC simulation will be on a separate PLC or PC that is communicating with the PLC under test, you can run the PLPS on the same PLC, with some restrictions, which we will discuss. Passive loopback process simulation (PLPS) is the answer. So, how can a CSI address the testing dilemma if the budget does not include a simulation software package? This effect presents itself not only on huge process control projects with plentiful resources and visibility, but also on small tasks with minimal or no additional equipment or infrastructure. This savings in time not only feeds the bottom line, but also mitigates the risk associated with a poor startup, which negatively affects prestige-a more important parameter than cost. To our knowledge, no formal study has tested the actual time saved by making a software correction in the lab versus making that same correction in the field, but there is ample evidence to suggest those savings can be immense. To many, process simulation is simply a luxury. This software can be prohibitively expensive, requiring a very steep learning curve and having associated maintenance fees and recurring costs. One alternative is to procure and configure simulation software to emulate field devices and complex control schemes. In addition, from a validation point of view, the tests are not repeatable, as they depend on the user to input values and responses.
Although most modern PLC software has tools to simulate I/O, to force/clear internal bits, and to set analog values through animation tables, employing those techniques to perform a realistic test of the PLC program can be unwieldy and time consuming. The vast majority of projects executed by CSIs are small, with a fast turnaround and limited budget, resources, and schedule. The CSI often develops software in the lab and implements it in the field.
Loop back device Offline#
Programmable logic controller (PLC) software development may be done completely offline (static), or in a lab setting with only a PLC rack, a CPU, a power supply, and some kind of communications port.
Loop back device full#
CSIs often work remotely, without having the actual equipment to manipulate, or even a full complement of I/O racks and modules. One challenge faced by all control systems integrators (CSIs) is the prospect of testing their software before deployment.
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