Siemens SIMATIC S7-200 System Manual page 44

S7-200 Programmable Controller System Manual
High-Speed Counters: HC
The high-speed counters count high-speed events independent of the CPU scan. High-speed
counters have a signed, 32-bit integer counting value (or current value). To access the count
value for the high-speed counter, you specify the address of the high-speed counter, using the
memory type (HC) and the counter number (such as HC0). The current value of the high-speed
counter is a read-only value and can be addressed only as a double word (32 bits).
Format:
Accumulators: AC
The accumulators are read/write devices that can be used like memory. For example, you can use
accumulators to pass parameters to and from subroutines and to store intermediate values used
in a calculation. The S7-200 provides four 32-bit accumulators (AC0, AC1, AC2, and AC3). You
can access the data in the accumulators as bytes, words, or double words.
The size of the data being accessed is determined by the instruction that is used to access the
accumulator. As shown in Figure 4-8, you use the least significant 8 or 16 bits of the value that is
stored in the accumulator to access the accumulator as bytes or words. To access the
accumulator as a double word, you use all 32 bits.
For information about how to use the accumulators within interrupt subroutines, refer to the
Interrupt Instructions in Chapter 6.
Format:
Figure 4-8 Accessing the Accumulators
30
HC[high-speed counter number]
AC[accumulator number]
AC2 (accessed as a byte)
AC1 (accessed as a word)
AC3 (accessed as a double word)
MSB
31 24 23
Most significant
Byte 3 Byte 2
HC1
AC0
MSB
7
MSB
15 8 7
Most significant Least significant
Byte 1 Byte 0
16 15 8 7
Least significant
Byte 1 Byte 0
LSB
0
LSB
0
LSB
0