6 -2 I /O R ACK E NCLOSURES AND T ABLE M APPING

6 -2 I /O R ACK E NCLOSURES AND T ABLE M APPING

An I/O module is a plug-in–type assembly containing circuitry that commu- nicates between a PLC and field devices. All I/O modules must be placed or inserted into a rack enclosure, usually referred to as a rack, within the PLC (see Figure 6-3). The rack holds and organizes the programmable controller’s I/O modules, with a module’s rack location defining the I/O address of its connected device. The I/O address is a unique number that identifies the input/ output device during control program setup and execution. Several PLC manufacturers allow the user to select or set the addresses (to be mapped to the I/O table) for each module by setting internal switches (see Figure 6-4).

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S ECTION Components The Discrete C HAPTER 2 and Systems

Input/Output System 6

Figure 6-4. Internal switches used to set I/O addresses.

A rack, in general, recognizes the type of module connected to it (input or output) and the class of interface (discrete, analog, numerical, etc.). This module recognition is decoded on the back plane (i.e., the printed circuit board containing the data bus, power bus, and mating connectors) of the rack.

The controller’s rack configuration is an important detail to keep in mind throughout system configuration. Remember that each of the connected I/O devices is referenced in the control program; therefore, a misunderstanding of the I/O location or addresses will create confusion during and after the programming stages.

Generally speaking, there are three categories of rack enclosures:

• master racks • local racks • remote racks

The term master rack (see Figure 6-5) refers to the rack enclosure containing the CPU or processor module. This rack may or may not have slots available for the insertion of I/O modules. The larger the programmable controller system, in terms of I/O, the less likely the master rack will have I/O housing capability.

A local rack (see Figure 6-6) is an enclosure, which is placed in the same area as the master rack, that contains I/O modules. If a master rack contains I/O modules, the master rack can also be considered a local rack. In general, a local rack (if not a master) contains a local I/O processor that sends data to and

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32 Local I/O

Figure 6-5. Master racks (a) without I/O modules and (b) with I/O modules.

Master Rack

Local Rack

Local I/O

I/O Modules

Memory Figure 6-6. Local rack configuration.

from the CPU. This bidirectional information consists of diagnostic data, communication error checks, input status, and output updates. The I/O image table maps the local rack’s I/O addresses.

As the name implies, remote racks (see Figure 6-7) are enclosures, contain- ing I/O modules, located far away from the CPU. Remote racks contain an I/O processor (referred to as a remote I/O processor) that communicates input and output information and diagnostic status just like a local rack. The I/O addresses in this rack are also mapped to the I/O table.

The rack concept emphasizes the physical location of the enclosure and the type of processor (local, remote, or main CPU) that will be used in each particular rack. Every one of the I/O modules in a rack, whether discrete, analog, or special, has an address by which it is referenced. Therefore, each terminal point connected to a module has a particular address. This connec-

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Main Location Master Rack

Remote I/O Processor

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Remote Rack #1

Remote Rack #2

Remote I/O

Remote I/O

Figure 6-7. Remote rack configuration.

tion point, which ties the real field devices to their I/O modules, identifies each I/O device by the module’s address and the terminal point where it is connected. This is the address that identifies the programmed input or output device in the control program.