Publication 1763-RM001C-EN-P - October 2009
476 System Status File
Free Running Clock
This register contains a free running counter. This word is cleared (0)
upon entering an executing mode.
Bits in status word 4 can be monitored by the user program. The bits turn
on and off at a particular rate (cycle time). The On/Off times are identical,
and are added together to determine the cycle time.
S:4 Free Running Clock Comparison for SLC 500 and MicroLogix Controllers
The Free Running Clocks in the SLC 500 and MicroLogix controllers
function the same, but have different resolutions. The resolution of the
Free Running Clock depends upon which controller you are using.
• SLC 500 and MicroLogix 1000: 10 ms/bit (0.010 seconds/bit)
• MicroLogix 1100, MicroLogix 1200 and MicroLogix 1500: 100 µs/bit
(0.0001 seconds/bit)
The following table illustrates the differences.
Address Data Format Range Type User Program Access
S:4 binary 0 to FFFF status read/write
Free Running Clock Cycle Times (all Times are in Seconds)
Bit SLC 500 and MicroLogix 1000 MicroLogix 1100, MicroLogix 1200 and
MicroLogix 1500
On/Off Time Cycle Time On/Off Time Cycle Time
S:4/0 0.010 0.020 0.0001 0.0002
S:4/1 0.020 0.040 0.0002 0.0004
S:4/2 0.040 0.080 0.0004 0.0008
S:4/3 0.080 0.160 0.0008 0.0160
S:4/4 0.160 0.320 0.0016 0.0320
S:4/5 0.320 0.640 0.0032 0.0640
S:4/6 0.640 1.280 0.0064 0.1280
S:4/7 1.280 2.560 0.0128 0.2560
S:4/8 2.560 5.120 0.0256 0.5120
S:4/9 5.120 10.240 0.0512 0.1024
S:4/10 10.240 20.480 0.1024 0.2048
S:4/11 20.480 40.960 0.2048 0.4096
S:4/12 40.960 81.92 0.4096 0.8192
S:4/13 81.92 163.84 0.8192 1.6384
S:4/14 163.84 327.68 1.6384 3.2768
S:4/15 327.68 655.36 3.2768 6.5536
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