Copyright Goodheart-Willcox Co., Inc. Chapter 6 PLC Programming 101 Goodheart-Willcox Publisher Figure 6-21. PLC status file. • Timer (T4)—The file that holds information related to the number of timers used in the ladder logic program and the contents of the timer instructions. Depending on the size of the PLC RAM, there may be timers with addresses from T4:0 to T4:255. This means that, theoretically, a PLC ladder diagram can have access to 256 timers. In practice, the number of timers is limited by the amount of RAM memory in the PLC. • Counter (C5)—The file that holds information related to counters, which can have any address from C5:0 to C5:255. The number of counters is also limited by the amount of RAM memory in the PLC. • Control (R6)—A file that holds 256 words (R6:0 to R6:255) used as control registers for advanced PLC instructions. Control registers are used with sequencer and shift register instructions and hold valuable information related to the sequencer and bit shift instructions. • Integer (N7)—A file that 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. A B Goodheart-Willcox Publisher Figure 6-22. Integer files. A—RSLogix 500 software from Rockwell International. B—LogixPro software from The Learning Pit.