Advantech PCI-1784 specifications Counter modes, Single-ended vs. differential input

Single-ended vs. differential input

Most encoders produce square wave signals with TTL levels. Industrial systems often use encoder with differential signals, i.e. channel A and B and their complements. Differential signals can reduce sensitivity to noise and allow longer transmission distances. Encoders may also produce a third signal once per revolution know as the index or marker. The encoder interface can use the index signal to reset the counter, allowing you to monitor the position within the current revolution.

D.2 Counter modes

The following table shows the maximum input rate for each input mode and system clock rate. Values are given for each system clock frequency.

Mode		Maximum input rate
	8MHz	4MHz	2MHz	1MHz
Quadrature x1, x2, x4	2 MHz	1 MHz	500 KHz	250 KHz
2-pulse	8 MHz	4 MHz	2 MHz	1 MHz
Pulse/direction	8 MHz	4 MHz	2 MHz	1 MHz

Counter modes are as follows:

Quadrature input counter mode

Quadrature input consists of two square wave inputs (A and B)which are 90° out of phase. The PCI-1784 counts square wave transitions and determines the direction by comparing whether channel A is leading channel B or vice versa.

There are three different counting methods in quadrature input mode:

x1 The counter will increment (or decrement) the counter whenever a rising edge occurs on input channel A.

x2 The counter will increment (or decrement) whenever a rising or falling edge occurs on input channel A.

x4 The counter will increment (or decrement) whenever a rising or falling edge occurs on input channel A or B.

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