E NCODER /C OUNTER I N T E R FA C E S

E NCODER /C OUNTER I N T E R FA C E S

Encoder/counter modules interface encoders and high-speed counter devices with programmable controllers. This type of module operates independently of the processor and I/O scan. An encoder/counter module is an integral part of a programmable controller system when it is used in applications requiring position information. Such applications include closed-loop positioning of machine tool axes, hoists, and conveyors, as well as cycle monitoring of high-speed machines, such as can-making equipment, stackers, and forming equipment.

There are two types of encoder/counter interfaces: absolute and incremental. Absolute encoders provide an angular measurement of the shaft. They provide this angular position (expressed in BCD, binary, or Gray code) in parallel to the encoder interface module. Incremental encoders measure shaft

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Serial Communication Interfacing 8

rotation over distance by outputting a fixed number of pulses per shaft rotation. The module provides two pulse signals that have a 90 ° phase difference (quadrature); it then determines the direction of rotation by sensing which of the two pulse channels is the leading waveform. Incremental encoders provide a marker, or index, channel that sends a pulse for every shaft revolution. This marker, which is an input to the module, can be used in conjunction with the module’s limit switch channel input to establish a home position along the encoder’s measurements. When the encoder inter- face is used in a counter configuration, however, only one input channel can

be connected to a device that provides a pulse count. During operation, an encoder/counter module (in incremental encoder mode)

receives two pulse channel inputs that are counted and compared with a user- specified preset value. The interface may have one or two output lines available, which are energized once the incoming pulses are equal to, greater than, or less than the preset values. The input channels and output lines available are generally rated for TTL or for 12–48 VDC. The maximum input pulse frequency that an encoder/counter interface can properly count ranges between 50 and 60 kHz.

The communication between an encoder/counter interface and the processor is bidirectional. The module accepts the preset value and other control data from the processor and transmits values and status data to the PLC memory. The interface also lets the PLC know when the marker and limit switch are both energized, indicating a home position. On the other hand, the processor’s control program, which tells the module to operate the outputs according to the count value received, enables the output controls. The control program also enables and resets the counter operation.

Typically, the length between the module and the encoder should not exceed

50 feet, and shielded cables should be used. Since encoder/counter modules have both inputs and outputs, they have isolation between the input and output circuits, as well as between the control logic and both I/O circuits. The use of separate power supplies, which must be provided by the user, enhances this isolation. Figure 8-19 shows the typical connections for an incremental encoder configuration.