Page 712 - Mechatronics with Experiments

698 MECHATRONICS
3. and a CPU module (or a scanner card if PLC is used in conjunction with a master
controller, i.e., PC),
4. as many I/O modules as needed by specific application.

The slots form the electrical interface between the I/O modules and the bus of the PLC
in the backplane. The bus consists of four major group of lines: power lines, address lines,
data lines, and control lines. The end-user does not need to be concerned with the details
of the bus since the interface between the CPU and all of the I/O modules supported for a
given PLC is already worked out and cannot be modified by the user. The real-time kernel
on the PLC and the user program are stored in the memory. The memory can be ROM (read-
only memory), EPROM (electrically programmable ROM), EEPROM (erasable electrically
programmable ROM), or battery backed RAM (random access memory) type.
Each I/O point on each unit must have a unique address on the PLC bus. The I/O
address is generally determined based on the

rack number,
slot number, and

channel number.
Typically, there can be up to three to five racks supported by one PLC. In each rack,
there are 4 to 15 slots. In each slot, there can be a single I/O module. For instance, the address
of a 16-point discrete input module on the main rack, slot number 3, would be determined by
the address code Rack-Slot-IO Channel: 1-3-n, where n is 1 to 16 representing the 16 I/O
channels on the module. Similarly, the I/O modules that have analog signal interfaces
map their I/O values into the memory of the PLCs. Notice that a PLC with 3 racks,
12 slots in each rack, and supporting 16 discrete I/O in each slot, can support a total of
3 × 12 × 16 = 576 discrete I/Os. Similarly, the same PLC can also support the following
combination of discete and analog I/Os:
1. 8 channels of 16-bit A/D converter (8 × 16 = 128 bits),
2. 8 channels of 16-bit D/A converter (8 × 16 = 128 bits),
3. 320 discrete I/O channels (320 bits).
It was noted above that one of the main advantages of PLCs is the fact that a great
variety of I/O interface modules are available in standard form. A hardware interface of
a simple discrete I/O module has the same difficulty as the interface of a special purpose
I/O module. They all snap-on to one of the slots on the PLC rack. This standard hardware
interface proves to be a very important asset. The I/O data associated with the I/O modules
are memory mapped to the CPU’s address space. Below is a list of I/O interface modules
available for most PLCs.

1. Discete input modules for DC and AC type signals.
2. Discrete output modules for DC and AC type signals. Each discrete I/O module
typically contains 8, 16, or 32 I/O points.
3. Analog input modules (ADC with various voltage OR current range and resolution,
i.e., 0–5 VDC, 0–10 VDC, −10 to 10 VDC, 4–20 mA, 0–10 mA ranges, 10-bit,
12-bit, 16-bit resolutions).
4. Analogoutputmodules(DACwithvariousvoltagesandcurrentrangeandresolution).
5. Timer and counter modules (hardware timers, pulse and event counters). A hardware
timer module can be programmed to generate an input to the PLC as well as to
generate an output to a device when a time period is passed. The beginning (trigger)