Modernization is the process of moving a machine’s or process’s control over from traditional relay logic to a programmable controller. The control logic is housed in traditional hardwired relay panels. They can be difficult to maintain, including contact chatter and contact welding. The machine’s performance can be improved by switching to a PLC. It also allows for better control. Actually, the “new” program for programming the machine’s controller is based on the original hardwired system’s instructions and controls.
This section will show you how to set up and configure a PLC program. Each module has four I/O points in the controller’s I/O structure. There are eight racks (0-7), each with eight slots or groups where modules can be placed. This configuration is illustrated in Figure 22.
Figure 22: Example PLC configuration.
The PLC can handle four-channel analog input modules. These can be placed anywhere in the slot. If analog I/O modules have been used, they cannot be placed in the same slot as discrete I/O. Multiplexed registers can be accepted by the PLC. Multiplexed modules have two slots and provide enable (select), lines for I/O devices.
Addresses 000 to 777 octal are input and output device connections that have been mapped to I/O tables. The address’s first digit is the rack number. The second digit is the slot and the third digit the terminal connection. The PLC determines whether the slot has an input or output.
Point addresses 10008 to 27778 can be used as internal outputs. Register storage begins at register 30008 and ends with register 47778. There are two types of counter and timer formats: ladder format and block format. However, all timers need an internal output to indicate the ON-delay output. Block format timers put a “T” in front of heir internal output address. Ladder format timers add a “T”, while block format timers specify that address in the block’s output coil.
For discrete inputs, addresses 0008 through 0278 will be used in the following sections. Addresses 0308 through 0478 will be used for outputs. When possible, analog I/O will go in the last slot on the master rack (0). These examples will show you that sometimes, the assignment of registers and internals can be done simultaneously with the programming stages.