233
The overseeing time is fixed at 3.0 ms. The input refresh time would be as
follows for the five 16-point Input Units and two 32-point Input Units con-
trolled by the PC:
16 points
(16 points x 5) + (32 points x 2)
x 25
µ
s = 0.23 ms
The output refreshing and Unit servicing time equals the I/O bus check time
plus the output refresh time plus the peripheral device and Link Unit servicing
time. The I/O bus check time is fixed at 0.8 ms. The output refresh time
would be as follows for the five 16-point Output Units and two 32-point Out-
put Units controlled by the PC:
16 points
(16 points x 5) + (32 points x 2)
x 18
µ
s = 0.16 ms
The basic peripheral device and Link Unit servicing time would be 1.5 ms for
the Programming Console and 0.5 ms each for a Host Link Unit and two PC
Link Units. This time, a total of 3 ms, is required even though not all of these
Units are mounted to the PC. The total output refreshing time, including Unit
servicing time computed above, is thus 0.8 ms + 0.16 ms + 3 ms = 3.96 ms.
Because this is considerably less than the program execution time, which is
12 ms (0.6
µ
s/instruction times 20,000 instructions) it is necessary to com-
pute the number of times that peripheral device and Link Unit servicing would
be repeated to arrive at the actual output refreshing and Unit servicing time.
Subtracting 3.96 ms from 12 ms gives us 8.04 ms remaining before program
execution is completed. This would be enough time to complete two more
cycles of Peripheral Device and Link Unit servicing and still leave 2.04 ms to
service the Programming Console (1.5 ms) and two more Link Units (0.5 ms
each). The total output refreshing and Unit servicing time would thus be as
follows:
3.96 ms + (2 x 3 ms) + 1.5 ms + 0.5 ms + 0.5 ms = 12.46 ms
The cycle time would thus be: 3.0 ms + 12.46 ms + 0.23 ms = 15.69 ms
6-2-2
PC with Link Units
Here, the cycle time is computed for a C2000H Duplex System with three
16-point Input Units, two 16-point Output Units, a Remote I/O Master Unit, a
Host Link Unit, and a PC Link Unit on the CPU I/O Rack and two 16-point
Input Units, two 32-point Input Units, two 16-point Output Units, and two
32-point Output Units on an Optical Slave Rack. In this PC configuration,
there is also a Programming Console mounted to the CPU that needs to be
taken into consideration. The PC configuration for this could be as shown
Calculating Cycle Time
Section 6-2
