Page 710 - Mechatronics with Experiments

696 MECHATRONICS
Main rack (CPU rack)

I/O modules
CPU Power supply
Expansion I/O racks
module module

I/O modules

I/O modules
FIGURE 9.1: The PLC hardware configuration
and components: (a) Different PLC catagories in
terms of their number of I/O capabilities and CPU
speed. (b) A PLC CPU rack and three I/O
I/O modules
expansion racks.
the application needs changing, additional I/O modules can be added by simply inserting
them into the available slots on the main rack or the expansion rack. Each I/O module
occupies a finite number of memory in the PLC’s memory space. For instance, a 16-point
discrete input module occupies a 2-byte space in the PLC’s memory space. A four-channel
12-bit analog to digital (ADC) converter occupies 4 × 12 = 48 bit = 6 bytes of memory

PC
- Scans I/O
- Control software

Master
scanner

Scanner
RS-232
module
PLC
- I/O interface
- scanner module
maps I/O to PC scanner
- no local control logic PS

I/O modules
RS-232
FIGURE 9.2: Typically, a PC is used as an offline program development, as well as an online
debugging and monitoring tool (i.e., using the RS-232 serial interface). The PLC is used as the
“brains” of the control system by implementing the control logic, and provides the means for
I/O interface. In another mode (using master and slave scanner modules), the PC is involved in
the real-time control logic implementation. The PLC merely acts as the I/O interface device, and
maps the I/O to the communication bus between the PLC and PC using a scanner (slave) card.