Chapter 6 PLC Programming 117
Control (R6):
File that holds the
contents of the control
registers.
number of counters is also limited due to the lack of RAM
memory in the PLC.
• Control (R6)—The control file (R6) holds 256 words (R6:0
to R6:255) that are used as control registers for advance PLC
instructions such as bit shift and sequencer instructions. Control
registers are used with sequencer and shift register instructions.
Therefore, registers R6:0 to R6:255 can hold valuable information
related to the sequencer and shift register instructions.
• Integer (N7)—Integer file (N7) holds 256 words (N7:0 to N7:255)
that can be used as temporary storage integer registers. Several
16-bit integer registers with addresses N7:0 through N7:255 are
available in the SLC 500 series Allen-Bradley PLC systems. See
Figure 6-22. These integer registers are used either as holding
registers for instructions such as sequencers, or simply as storage
registers. Double-click the N7 icon to check and change the
value of these registers.
• Bit (B3 and B10)—Bit file B3 and B10 each hold 256 words
(B3:0 to B3:255 and B10:0 to B10:255). These registers are used
by advance PLC instructions such as bit shift and sequencer
instructions. All Allen-Bradley SLC 500 series Programmable
Logic Controllers have 16-bit registers. For example, B3:0 has the
bits B3:0/0 to B3:0/15. B3:0 represents a 16-bit word, while B3:0/0
or B3/0 represents the least significant bit in the B3:0 word. These
bits can be used as internal coils or contacts in a PLC ladder logic
diagram. There are 256 bits addressed as B3/0 through B3/255
available in the SLC 500 series PLC. These bits are used for
internal PLC coil or contact addresses. A programmer can also
use bits addressed as B10:0 through B10:255. Therefore, 512 bit
addresses labeled B3:0 through B3:255 and B10:0 through B10:255
are available in the SLC 500 series PLC systems. Note that the B3
data file has separate data file icons.
Figure 6-21. PLC status file.
Integer (N7):
File that holds the integer
numbers.
Bit (B3 & B10):
File used for internal
control relay contacts.