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5-4
PROGRAMMING AND INTERFACING
Using the Trigger Outputs
The Trigger I/Os are interfaced directly to the data lines of the 1750 microprocessor. The 1750 may
be triggered externally by providing a low TTL level with external circuitry or by shorting the center
conductor of the trigger input to its shield for a minimum of 10 ms with a relay or other device.
The trigger output may be read by any TTL compatible circuitry that meets the signal level
requirements as defined in Table 5.1. The Trigger output is normally in the high state until a read cycle
has completed. At this time the 5 VDC signal will go low for a minimum of 4.25 ms, then it returns to
a high state again until the next read cycle has completed.
If the Trigger output must be read by a PLC, it is recommended that a buffer be used to interface the
1750 trigger output to the PLC input. This can be done through optical isolation or through an open
collector configuration. Since additional buffer circuitry may not be practical for some PLC applications,
the following procedure provides a possible work around:
1. Using an external controller, (PC), send a command to the 1750 to initiate a reading cycle. The
1750 is assumed to be operating in either absolute or % comparator mode.
2. Upon the completion of the test cycle and acknowledging that the controller has received a new
reading from the 1750, send a command to change the 1750 display mode to resistance mode.
Changing the 1750 display mode from comparator mode to resistance mode will disable the
comparator outputs and return their state to either normally open or normally closed outputs.
These comparator outputs should be tied to the PLC inputs.
3. Create a ladder rung in the PLC that becomes true when all three comparator states become equal.
When this condition is met, the PLC detects that the reading cycle has been completed confirming
a reading complete condition.
4. Send a command to return the 1750 display mode back to one of the comparator modes. This will
enable the comparator outputs and create a reading in process condition for the PLC.
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