GFK-0467K Chapter 4 Series 90-30/20/Micro Instructions Set 4-137
4
SVCREQ #2: Read Window Values Use SVCREQ function #2 to obtain the current window mode time values for the programmer
communications window, the system communications window, and the background task window.
Note
Of the CPUs discussed in this manual, Service Request 2 is supported only
by 90-30 CPUs, beginning with Release 8.0.
There are three modes for each window:
Mode Name Value Description
Limited Mode 0The execution time of the window is limited to its respective
default value or to a value defined using SVCREQ function #3
for the programmer communications window or SVCREQ
function #4 for the systems communications window. The
window will terminate when it has no more tasks to complete.
Constant Mode 1Each window will operate in a RUN TO COMPLETION mode, and
the PLC will alternate among the three windows for a time
equal to the sum of each window’s respective time value. If
one window is placed in CONSTANT mode, the remaining two
windows are automatically placed in CONSTANT mode. If the
PLC is operating in CONSTANT WINDOW mode and a
particular window’s execution time is not defined using the associated
SVCREQ function, the default time for that window is used in the
constant window time calculation.
Run to Completion
Mode 2Regardless of the window time associated with a particular
window, whether default or defined using a service request
function, the window will run until all tasks within that window are
completed.
A window is disabled when the time value is zero.
The parameter block has a length of three words:
High Byte Low Byte
Programmer Window Mode Value in ms address
System Communications Window Mode Value in ms address + 1
Background Window address + 2
All parameters are output parameters. It is not necessary to enter values in the parameter block to
program this function. Output values for all three windows are given in milliseconds.