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This does allow faster data transmission rates, but the connectors and cables become much larger, more expensive and less flexible. These interfaces still use handshaking to control data flow.

These interfaces are common for computer printer cables and short interface cables, but they are uncommon on PLCs. A list of common interfaces follows.

Centronics printer interface -These are the common printer interface used on most personal computers. It was made popular by the now defunct Centronics printer company.

GPIB/IEEE-488 -(General Purpose Instruments Bus) This bus was developed by Hewlett Packard Inc. for connecting instruments. It is still available as an option

plc serial -27.14

27.4 DESIGN CASES

27.4.1 PLC Interface To a Robot

Problem: A robot will be loading parts into a box until the box reaches a prescribed weight. A PLC will feed parts into a pickup fixture when it is empty. The PLC will tell the robot when to pick up a part and load it into the box by passing it an ASCII string, "pickup".

Figure 27.12 Box Loading System

Solution: The following ladder logic will implement part of the control system for the system in Figure 27.12.

plc serial -27.15

Figure 27.13 A Box Loading System

27.5 SUMMARY

* Serial communications pass data one bit at a time.
* RS-232 communications use voltage levels for short distances.

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