Rockwell Automation Publication 1734-UM007E-EN-P - October 2015
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Set and Operate Your Module
Chapter 4
Operation of the Data Latch
and Comparator Features
Read this section for information about operation of the Data Latch and
Comparator features.
Data Latch
See the table for a listing of values for DeviceNet name, Logix Designer or
RSLogix 5000 tag, and Logix Designer or RSLogix 5000 field name.
SSI sensor positions can be stored with operation of the digital input (I1). The I1
input is sourcing (IEC Type 3 compatible); thus grounding the input turns it
ON. The yellow I1 LED on the front of the SSI module also illuminates when
the I1 input is ON.
The I1 input is edge-sensitive: the module can react to rising or falling edges of
I1. Choosing the falling data latch value provides OFF-to-ON data latching, and
choosing the rising value provides ON-to-OFF latching. Choose the Both value
if you need OFF-to-ON and ON-to-OFF data latching simultaneously.
With one of the Latch Input modes active, watch the module Status Word (status
bytes 0 and 1) for I1 input operation. When I1 is ON, bit I1 in Status Byte 0 (bit
0 of Status Byte 0) is 1. If I1 is OFF, the I1 bit is 0. When the I1 input is activated
corresponding to the Latch Input mode, the current SSI sensor value is stored in
produced bytes 4 to 7.
The LHON bit (bit 4 of Status Byte 1) will be set to 1. To unlatch the stored
value, toggle (set to 1 for about 0.5 ms, then back to 0) the LACK bit, which is
bit 0 of the Master Ack Byte. The Master Ack Byte is consumed Byte 0 sent by
the master to the 1734-SSI module. Once the LACK bit is toggled, LHON is set
back to 0 and the produced bytes 4 to 7 return to 0 as well. You can also unlatch
stored data by turning the Latch Input parameter OFF through the
configuration. The LHON bit returns to 0 as well when the Data Latch is turned
OFF through the configuration.
Data Latch
DeviceNet Name
Logix Designer/RSLogix 5000 Tag
Logix Designer/RSLogix 5000
Field Name
Bit 0 of Status Byte 0
Input
InputStatus
LHON
Input
DataLatched
LACK
Output
LatchAck