Objectives
This article provides information about the performance of the CP1W analog option boards (CP1W-ADB21, CP1W-DAB21V and CP1W-MAB221). Understanding the factors that influence the operation of these devices can help in the selection of equipment and quality of the application.
Introduction
CP-series analog option boards rely on PLC peripheral process servicing which can be affected by several PLC functions. It is important to consider these influences when selecting an analog input/output device for the CP-series PLC, as there are many options. The CP-series PLC provides several ways to manage analog I/O signals:
- Built-in analog inputs on the CP1L-EL/EM CPU
- Analog option boards for the CP1L-EL/EM CPU
- Expansion units for the CP-series qualifying CPUs
- CJ1W analog units used with the CP1H CPU
Understanding the following factors described in this article can aide in the selection of the appropriate analog device and increase performance of the CP1W analog option board.
- PLC cycle time
- Peripheral servicing
- Baud rate
Before describing the details of the cycle time and peripheral servicing, the following flowchart shows the overall operation of the CPU unit for a general understanding of how these functions are executed during normal operation.
Content
PLC Cycle Time
The PLC cycle time affects when analog values are converted and refreshed, either for input, output or both. This refresh time is >6ms for the option board and will increase as PLC cycle time increases. The analog-to-digital and/or digital-to-analog conversion occurs once per PLC cycle so variations in program size, structure and instruction use will have a direct effect on the refresh time of the option board. Consider the following methods to decrease the cycle time if this is causing prolonged analog option board refreshing:
- Separate instructions not being executed into different tasks
- Consider using jump instructions for areas in the task that are not executed
- Prohibit cycle refreshing with Special I/O units that do not require exchange of cycle data
Peripheral Servicing
Peripheral servicing involves servicing non-scheduled events for external devices. This includes both events from external devices and service requests to external devices. Most peripheral servicing involves FINS commands. The specific amount of time set in the system is allocated to each type of servicing and executed every cycle. If all servicing cannot be completed within the allocated time, the remaining servicing is performed the next cycle.
Ethernet port, serial port, and communications port servicing is allocated 8% of the previous cycle time by default (the default can be changed) for each service. If servicing is separated over many cycles, delaying completion of the servicing, set the same allocated time (same time for all services) rather than a percentage under execute time settings in the PLC Setup.
Ethernet and Serial port servicing can cause option board sampling rates to vary if communication traffic loads change during operation. Adjusting the peripheral service time to a fixed value rather than 8% of the total cycle time (default) can stabilize the option board sample rate.
More details on peripheral servicing can be found in the attached manual, Sysmac CP Series CP1L-EL/EM CPU Unit Operation Manual (Cat. No. W516).
Baud Rate
Since analog information is transferred to and from the option board via serial communications, the baud rate setting for the option board slot can have an influence on refresh time. For optimum performance, ensure the baud rate setting of the occupied option board slot is 115200bps (Toolbus mode).
*Note: When the appropriate port dip switch setting is ON (Auto), the port is automatically set at the maximum 115200bps in Toolbus mode.
Other Tips To Improve Analog Data Processing
- Consider using an analog expansion unit with the I/O refresh (IORF) instruction
- Consider using the built-in analog input. This conversion time will be the same as the PLC cycle time and may be faster than option board conversion time. Be aware that this input has different specifications than an option board analog input. See the attached manual for details.
Summary
There are several factors that affect the sample rate of the CP-series analog option boards. The manual states that the sample rate is >6ms based on baud rate and PLC cycle time. Understanding these properties and how to manipulate them can help enhance the performance of an analog option board installed in the CP-series PLC.
*These details and references were made in January of 2014. Newer versions may operate and function differently. Contact your local Omron representative for more information.
