Factors to be considered in Purchasing a PLC

After the planning phase of the design, the equipment can be ordered. The decision will depend upon the basic criteria listed below.

·         Number of logical input and outputs.

·         Memory – often 1K and up. Need is dictated by size of ladder logic program.  A ladder element will take only a few bytes, and will be specified in manufacturers documentation.

·         Number of special I/O modules- when doing some exotic application, a large number of specual add-on cards may be required.

·         Scan Time- Big programs or faster processes will require shorter scan times.  And, the shorter the scan time, the higher the cost.  Typical values for this are 1 microsecond per simple ladder instruction.

·         Communications – Serial and networked connections allow the PLC to be programmed and talk to other PLCs. The needs are determined by the application.

·         Software – Availability of programmed software and other tools determines the programming and debugging ease.

The process of selecting a PLC can be broken into the steps listed below.

1.       Understand the process to be controlled.

·         List the number of types of input and outputs.

·         Determine how the process is to the controlled.

·         Determine special needs such as distance between parts of the process.

2.       If not already specified, a single vendor should be selected.  Factors that might be considered are, (Note: vendor research may be needed here)

·                                             Manuals and documentation

·                                             Support while developing programs

·                                             The range of products available

·                                             Shipping times for emergency replacements

·                                             Training

·                                             The track record for the company

·                                             Business practices (billing upgrades/obsolete product, etc.)

3.       Plan the ladder logic for the controls.

4.       Count the program instruction.  Use the instruction, times and memory requirements for each instruction to determine if the PLC has sufficient memory, and if the response time will be adequate for the process.

5.       Look for special program needs and check the PLC model. (e.g. PID)

6.       Estimate the cost for suitable hardware, programming software, cables manuals, training etc., or ask for a quote from a vendor.