Programming instructions

2.6 Special Function Modules

All of the modules that have been discussed thus far have been DISCRETE I/O

modules. The inputs or outputs are either ON or OFF. This is acceptable if all

the inputs in the PLC system are switches or simple sensors, and the outputs are

or lights to turn off and on. If it is necessary to monitor or control a temperature,

talk to a network, or control a positioning module, it is necessary to use a Special

Function Module (referred to as SFM) to accomplish these tasks. Each of these

modules has a range of internal addresses where the data is stored. The PLC

has dedicated commands to access this data.

Right Side Bus Analog Modules

There are 3 types: Analog Input modules, Analog Output modules and

Combination Analog Input/Output modules. All are used with I/O points that have

more states than just on or off. Examples of analog inputs would be a velocity

reading or pressure reading. An example of an analog output would be the

variable speed of a motor.

The input cards come in 2, 4, or 8 channels. These are the FX2N-2AD, FX2N-

4AD, and FX2N-8AD. The output cards come in either 2 or 4 channels. These

are the FX2N-2DA and FX2N-4DA. There are 2 combination cards. The FX0N-

3A has 2 input channels and 1 output channel. The FX2N-5A has 4 input

channels and 1 output channel. The FX3U family also has two dedicated analog

modules, the FX3U-4AD and FX3U-4DA, which are higher resolution and faster

processing modules.

All are based on varying current or voltage, usually –20 mA to +20 mA, 4-20 mA

–10 to + 10 V, as set by the programmer. Depending on the card, the module

receives from the PLC or from the input, a raw number (range varies by module

and type, see manuals for ranges), that is interpreted as the analog value read (if

an analog input) or the analog value to be sent out (if an analog output)

Programming Example

The PLC programmer wants to detect small changes in

pressure to control a chemical mixing process. He has a

sensor that has a range of 0 PSI to 300 PSI and generates a

voltage of –10V to +10V, and the FX2N-4AD module, which

sees –10V as the number -2000, and +10 V as +2000.

Given this information, the programmer knows that at 0 PSI

the sensor sends a voltage of –10V, which results in a value

of -2000 being written to the PLC. At 150 PSI, 0V is

generated, resulting in a value of 0 being written to the PLC.

At 300 PSI, the voltage is 10V and the PLC value is 2000. It

is the responsibility of the programmer to know what

pressure value equates to what PLC value, and to write the

program to scale that value accordingly.