Putting Together a Modular PLC Chapter 9 Putting Together a Modular PLC

Objectives Define rack, chassis, and baseplate and tell how or why they differ. Select the proper type of I/O to interface a specific input signal. Explain why power supply loading must be determined as a PLC system is configured.

PLCs Come in Two Styles The I/O of a fixed PLC is built in and not changeable. A modular PLC consists of user-selected I/O modules, a processor, a power supply, and a chassis.

Rack, Chassis, or Baseplate? Depending on the modular PLC manufacturer, the term used to identify the hardware device that holds all the modules, processor, and power supply may vary. Some use rack, chassis, or baseplate.

Module Installation into a Four-Slot SLC 500 Chassis (1 of 2) Image courtesy of Allen-Bradley, a Rockwell Automation business

Module Installation into a Four-Slot SLC 500 Chassis (2 of 2) The General Electric Series 90-30 incorporates the processor into the baseplate for the low-end modular PLCs. Baseplates come in five-slot and ten-slot models.

Series 90-30 PLC Model 331 CPU Image courtesy of GE Fanuc Automation

Removing a GE Series 90-30 I/O Module Image courtesy of GE Fanuc Automation

Local I/O Expansion When more I/O is required than a single chassis can hold, additional chassis can be added. Communications cable connects expansion chassis together. There is no processor in expansion chassis. A power supply is required in all chassis. The expansion cable distance is limited.

SLC 500 3-chassis Local Expansion Image courtesy of Allen-Bradley, a Rockwell Automation business

Remote I/O Allows greater cable distance between chassis Typically a serial link SLC 500 remote I/O maximum cable length 10,000 feet Baud rate of 57.6 K bits per second

SLC 500 Remote I/O Example (1 of 3) Image courtesy of Allen-Bradley, a Rockwell Automation business

SLC 500 Remote I/O Example (2 of 3) SLC 500 uses 1747-SN scanner in local chassis. SN scanner communicates with remote chassis. 1747-ASB communication module is required in each remote chassis.

SLC 500 Remote I/O Example (3 of 3) Remote I/O allows I/O to be “distributed” around the plant floor. PanelView operator interface terminals as well as variable frequency drives can be connected on a remote I/O link.

PLC Networking A single chassis PLC can be part of a larger factory-wide communication network. Each PLC has its own processor. Each device on the network is a node or station on the network. Each node must have a unique identifier known as the node address.

Advantages to Networking Multiple stand-alone PLCs networked together Share information between processors Share control of the process Operator interface devices input or display information from multiple PLC processors PLC control of other devices such as VFDs

Current SLC 500 Network Connectivity Remote I/O Data Highway-485 Data Highway Plus Control Net Device Net Ethernet

AB SLC 500 Data Highway-485 Network Image courtesy of Allen-Bradley, a Rockwell Automation business

Selection and Placement of I/O Modules Consider input and output signals Modules are divided into family groups. The appropriate input or output module is selected by determining incoming and outgoing signals. Match I/O signals to proper module.

Table for Determining Input and Output Types

I/O Module Selection from Input and Output Type

Power Supply Selection (1 of 2) Each modular PLC chassis, rack, or baseplate must have its own power supply. Power supply is designed to handle specific load. Power supply loading is dependent on modules installed.

Power Supply Selection (2 of 2) Proper power supply sizing will help avoid intermittent problems or power supply shutdown due to an overloaded power supply.

SLC 500 Power Supply Specifications Image courtesy of Allen-Bradley, a Rockwell Automation business

Determine Power Supply Loading Proper size power supply is selected to handle load placed upon it by the I/O module mix. Power supply loading can be manually calculated using manufacturers’ tables and module loading data. PLC programming software can provide power supply loading calculation feature.

RSLogix 500 Power Supply Loading

Installing an SLC 500 Power Supply Image courtesy of Allen-Bradley, a Rockwell Automation business

Series 90-30 Power Supply Image courtesy of GE Fanuc Automation