Are the inputs on the Allen-Bradley (A-B) 1769-L1xER high speed?

1769-L1xERLast week a colleague asked me if the inputs on the Allen-Bradley (A-B) 1769-L1xER were high speed inputs, like found on some MicroLogix.

I honestly had no idea, but I was on my way back to my home office so I told him I would look it up and get back to him.

What I found in my research was, when compared to typical the DC input modules like the 1769-IQ16, the 1769-L1's built-in I/O was indeed faster. To illustrate this comparison I have pasted the specs of both below:

1769-IQ16 Input Speed Specs:

1769-IQ16 Input Speed Specs from 1769-IN007

1769-L1xER Embedded Input Speed Specs:

1769 L1 Embedded Input Speed Specs from 1769-TD005

As you can see above, while the 1769-IQ16 has an on / off delay time of 8ms (what I would consider typical,) the 1769-L1 embedded I/O has a much faster 0.5ms on / off delay time – the same as 1734 point I/O modules like the 1734-IB4.

However, this isn't the whole story. While I/O delay time is important, with the CompactLogix and ControlLogix we must also consider if the I/O supports event based tasks, as well as the maximum supported RPI setting.

Dropping the 1769-L1 into RSLogix5000 v20 it quickly became apparent the embedded I/O can not trigger an event based task, as seen in the below screen shot:

1769-L1 Event Task

The final consideration was to find the fastest RPI supported by the L1's embedded I/O. While RSLogix5000 v20 does support a setting of 1ms RPI for the embedded I/O, the below excerpt from the L1 Tech Data sheet reveals that the true limitation is based on the number of modules (including embedded) in a system:

1769-L1 RPI Limits from 1769-TD005

In summary, the 1769-L1 family of CompactLogix processor's embedded I/O contains the same .5ms on / off delay times as many 1734 Point I/O modules of the same type. And while this is considerably faster than standard I/O modules, like the 1769-IQ16 with it's 8ms on / off delay, the speed advantage is quickly lost as you add more I/O modules to the 1769-L1 base system (with a full 8 module system limited to a maximum RPI of 8ms.)

Note: For more details on the 1769-L1 see the Allen-Bradley 1769-TD005 Technical Data Sheet here.

I hope this review of the speed of the embedded DC inputs on the 1769-L1 has been helpful. If you have any comments or questions please don't hesitate to leave them by using the “post a comment or question” link below.


Shawn Tierney
Automation Instructor and Blogger
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