ALLEN BRADLEY 1747-L551 PLC Processor

Selecting Your Hardware Components
2–3
Principles of Machine Control
You enter a logic program into the controller using the software. The
logic program is based on your electrical relay print diagrams. It
contains instructions that direct control of your application.
With the logic program entered into the controller, placing the
controller in the Run mode initiates an operating cycle. The
controller’s operating cycle consists of a series of operations
performed sequentially and repeatedly, unless altered by your
program logic.
service
comms
➃
output
scan
➂
➄
overhead
➀
input
scan
Operating Cycle
➁
program
scan
➀ input scan – the time required for the
controller to scan and read all input data;
typically accomplished within ms.
➁ program scan – the time required for the processor to
execute the instructions in the program. The program
scan time varies depending on the instructions used
and each instruction’s status during the scan time.
Important: Subroutine and interrupt instructions
within your logic program may cause
deviations in the way the operating cycle
is sequenced.
➂ output scan – the time required for the
controller to scan and write all output data;
typically accomplished within ms.
➃ service communications – the part of the operating cycle in
which communication takes place with other devices, such as
an HHT or a personal computer.
➄ housekeeping and overhead – time spent on memory
management and updating timers and internal registers.
Selecting Chassis
The chassis houses the processor and I/O modules. The power
supply mounts on the left side of the chassis. All components easily
slide into the chassis along guides formed in the chassis. No tools
are required to insert or remove the processor or I/O modules. A
maximum of three chassis (30 I/O slots) may be connected in an
SLC system.
There are four sizes of chassis that you can choose: 4-slot, 7-slot,
10-slot, and 13-slot. For chassis dimensions, see chapter 4.
Publication 1747-6.2