Allen-Bradley MicroLogix 1400 Instruction Manual page 564

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562 System Status File
Publication 1766-RM001A-EN-P - October 2008
Free Running Clock
Address Data Format
S:4 binary
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, MicroLogix 1400 and MicroLogix
1500: 100 µs/bit (0.0001 seconds/bit)
The following table illustrates the differences.
Free Running Clock Cycle Times (all Times are in Seconds)
Bit SLC 500 and MicroLogix 1000
On/Off Time
S:4/0 0.010
S:4/1 0.020
S:4/2 0.040
S:4/3 0.080
S:4/4 0.160
S:4/5 0.320
S:4/6 0.640
S:4/7 1.280
S:4/8 2.560
S:4/9 5.120
S:4/10 10.240
S:4/11 20.480
S:4/12 40.960
S:4/13 81.92
S:4/14 163.84
S:4/15 327.68
Range Type
0...FFFF status
MicroLogix 1100, MicroLogix 1200,
MicroLogix 1400 and MicroLogix 1500
Cycle Time On/Off Time
0.020 0.0001
0.040 0.0002
0.080 0.0004
0.160 0.0008
0.320 0.0016
0.640 0.0032
1.280 0.0064
2.560 0.0128
5.120 0.0256
10.240 0.0512
20.480 0.1024
40.960 0.2048
81.92 0.4096
163.84 0.8192
327.68 1.6384
655.36 3.2768
User Program Access
read/write
Cycle Time
0.0002
0.0004
0.0008
0.0160
0.0320
0.0640
0.1280
0.2560
0.5120
0.1024
0.2048
0.4096
0.8192
1.6384
3.2768
6.5536