snap ethernet-based i/o units protocols and programming guide

CHAPTER 2: OVERVIEW OF PROGRAMMING
Similarly, an off-latch is set when the value of a digital point changes from on to off, and it
remains set until cleared.
Latching is automatic and needs no configuration. Using memory map values, you can read the
on-latch or off-latch state of any digital point, and you can clear latches.
Counters (Digital Points)
This feature applies to digital points on I/O units with the following brains only:
SNAP-UP1-ADS
SNAP-B3000-ENET
On these I/O units, any digital input can be used as a counter. When configured, it will count the
number of times the input changes from off to on.
When a counter is configured, it is automatically activated.
Using memory map values, you can stop a counter if required and then start it again. You can also
clear a counter, so it begins counting again from zero. For example, to count the number of
widgets produced per shift, you would clear the counter at the start of each shift and read it at
the end of each shift.
Quadrature Counters (Digital Points)
I/O units with the following brains also support quadrature counters for quadrature encoder
devices:
SNAP-UP1-ADS
SNAP-B3000-ENET
SNAP-ENET-RTC
SNAP-WLAN-FH-ADS
SNAP-ENET-RTC
SNAP-WLAN-FH-ADS
A quadrature counter requires a SNAP quadrature input module, which is attached to the encoder
device. The module sends a pulse to the brain upon each change in quadrature state, and the
brain counts the pulses and keeps track of the direction and rotation. The counter counts up if
quadrature Point A leads Point B; it counts down if Point A lags behind Point B.
Optionally, you can assign a third digital input as an index for a quadrature counter. The index
automatically resets the count, and it shows what the count was when the index was triggered.
Counts are sometimes lost, due to noise or encoder problems, for example; with the index, you
can see whether the count varies too much.
Using a Simple Quadrature Counter
When you configure the points on the quadrature input module in ioManager or ioControl, the
simple quadrature counter feature (without an index) is automatically included.
SNAP Ethernet-Based I/O Units Protocols and Programming Guide 21