GE 90-30/20/Micro manual Sweep Time Calculation

2

Sweep Time Calculation

Table 2-1 lists the seven items that contribute to the sweep time of the PLC. The sweep time consists of fixed times (housekeeping and diagnostics) and variable times. Variable times vary according to the I/O configuration, size of the user program, and the type of programming device connected to the PLC.

Example of Sweep Time Calculation

An example of the calculations for determining the sweep time for a Series 90-30 model 331 PLC are shown in the table shown below.

The modules and instructions used for these calculations are listed below:

∙Input modules: five 16-point model 30 input modules.

∙Output modules: four 16-point model 30 output modules.

∙Programming instructions: A 1200-step program consisting of 700 boolean instructions (LD, AND, OR, etc.), 300 output coils (OUT, OUTM, etc.), and 200 math functions (ADD, SUB, etc.).

		Time Contribution
Sweep		wo/	w/	w/
Component	Calculation	Programmer	HHP	LM90
Housekeeping	0.705 ms	0.705 ms	0.705 ms	0.705 ms
Data Input	0.055 x 5 = .275 ms	0.275 ms	0.275 ms	0.275 ms
Program	1000 x 0.4 µ s* + 200 x 89 µ s** = 18.2 ms	18.2 ms	18.2 ms	18.2 ms
Execution
Data Output	0.061 x 4 = .244 ms	0.244 ms	0.244 ms	0.244 ms
Programmer	0.4 ms + programmer time + 0.6 ms	0 ms	4.524 ms	2.454 ms
Service
Non-	None in this example	0 ms	0 ms	0 ms
Programmer
Service
Reconfiguration	0.639 ms	0.639 ms	0.639 ms	0.638 ms
Diagnostics	0.048 ms	0.048 ms	0.048 ms	0.048 ms
PLC Sweep Time	Housekeeping + Data Input + Program	12.611 ms	17.135 ms	15.065 ms
	Execution + Data Output + Programmer
	Service + Non-Programmer Service +
	Diagnostics

*Refer to Table A-3, “Boolean Execution Speed,” in Appendix A for contact and coil execution speeds, which vary by CPU.

**Refer to Table A-1, “Instruction Timing,” for specific timing information. The figure used in this calculation, 89 microseconds, represents the average of the add, subtract, shift, used in the example.

Housekeeping

The housekeeping portion of the sweep performs all of the tasks necessary to prepare for the start of the sweep. If the PLC is in CONSTANT SWEEP mode, the sweep is delayed until the required sweep time elapses. If the required time has already elapsed, the OV_SWP %SA0002 contact is set, and the sweep continues without delay. Next, timer values (hundredths, tenths, and seconds) are updated by calculating the difference from the start of the previous sweep and the new sweep

2-6 Series 90-30/20/Micro Programmable Controllers Reference Manual– September 1998 GFK-0467K