Greetings brucechase ...
here’s something that I said in
this post from a related thread ...
my best guess right now (pocket change only) is that the (configurable) 1746-NI8 “works different” than the (non-configurable) 1746-NIO4I module - even when the configuration is set for NI4 operation ... at least that’s what I EXPECT to find ... anyway ... I’ll let you know what finally happens ...
my expectations were correct ... (I love it when that happens) ... here’s a quick rundown of the little experiment that I just worked through ...
Slot 0 = 1747-L542C processor - SLC-5/04 Series C - OS401
Slot 3 = 1746-NIO4I Series A combination analog module
Slot 4 = 1746-NI8 Series A FRN 1.0 analog input module - Class 3
both analog modules were set up and wired for “Current” inputs ... the 1746-NI8 module was configured with a data value of “2077” ... the analog input signal was supplied by a Fluke 787 Process Simulator ... the current signal was wired in series through both of the analog input modules ...
overview: the 1746-NIO4I module uses the “plain vanilla” scaling format commonly known as the “NI4” format ... specifically, a 4.00 mA input signal shows up as a data reading of 3277 ... and a 20.00 mA input signal shows up as a data reading of 16384 ... the 1746-NI8 module is “configurable” to handle different scaling ranges ... for the purpose of this test, the module was configured to use the “NI4 Data Format” ...
the issue which is the subject of this thread is as follows:
when the 1746-NIO4I module experiences an input signal which is LESS THAN 4.00 mA, the data reading from the module falls below 3277 ...
BUT ...
when the 1746-NI8 module experiences an input signal which is LESS THAN 4.00 mA, the data reading from the module does NOT fall below 3277 ...
similarly ...
when the 1746-NIO4I module experiences an input signal which is GREATER THAN 20.00 mA, the data reading from the module rises above 16384 ...
BUT ...
when the 1746-NI8 module experiences an input signal which is GREATER THAN 20.00 mA, the data reading from the module does NOT rise above 16384 ...
one specific cause for concern might be as follows ... suppose that a program has been written for use with a 1746-NIO4I module ... suppose that a couple of simple comparison rungs have been programmed to test the analog input signal - and to sound an alarm if the current signal is either too high or too low ... that program might work reliably ...
now suppose that a new 1746-NI8 module is to be substituted in place of the original 1746-NIO4I module ... a significant issue might develop if the “monitor/alarm” logic is not properly modified for use with the new module ... specifically, since the data from the new module will NOT fall outside of the expected range, the original comparison rungs will never detect a faulty signal ...
oops! ...
luckily there is a solution to the problem ... when the 1746-NI8 module is configured for operation in “Class 3” mode, the module returns an “extra” 8 words of data to the input table - in addition to the expected 8 words of “channel input data” ... each one of these “extra” words corresponds to one of the module’s analog input channels - and included in each “extra” word is an “Over-Range” bit - and an “Under-Range” bit ... the proper use of these two bits could serve the same purpose of the comparison rungs that we discussed earlier ...
so here’s the experiment that I ran ...
Test (1) = input signal too low ... I cranked in an analog input signal of 3.50 mA ...
Results: the data from the 1746-NIO4I module registered at 2867 ... (note that this IS below the “low-end” reading of 3277) ... the data from the 1746-NI8 module registered at 3277 ... (note that this is NOT below the “low-end” reading of 3277) ... the “Under-Range” bit (I:4.8/14) for the 1746-NI8 module had a status of 1 ... the “Over-Range” bit (I:4.8/13) for the 1746-NI8 module had a status of 0 ...
Test (2) = input signal ok ... I cranked in an analog input signal of 12.00 mA ...
Results: the data from the 1746-NIO4I module registered at 9826 ... the data from the 1746-NI8 module registered at 9828 ... the “Under-Range” bit (I:4.8/14) for the 1746-NI8 module had a status of 0 ... the “Over-Range” bit (I:4.8/13) for the 1746-NI8 module had a status of 0 ...
Test (3) = input signal too high ... I cranked in an analog input signal of 20.50 mA ...
Results: the data from the 1746-NIO4I module registered at 16786 ... (note that this IS above the “high-end” reading of 16384) ... the data from the 1746-NI8 module registered at 16384 ... (note that this is NOT above the “high-end” reading of 16384) ... the “Over-Range” bit (I:4.8/13) for the 1746-NI8 module had a status of 1 ... the “Under-Range” bit (I:4.8/14) for the 1746-NI8 module had a status of 0 ...
and I think that this answers the original question ... if not, please post again ... I’ll leave the same equipment set up for another day or two - just in case you want me to nail down anything else along the same lines ...