AB 1762-IQ8OW6 / MicroLogix 1100

[RonChownyk]

With the ML1100 being a Legacy product, this is something of an academic discussion.


I have an ML1100 with two expansion modules - 1762-IF2OF2 Analog module in slot 1 and a 1762-IQ8OW6 Discrete Combo module in slot 2.


If I do not add the IQ8OW6 to the IO Configuration, the CPU will happily go in to the Run mode WITHOUT creating a CPU Fault. The module addresses do not show up in the Data Files, and attempting to address its IO points in the program will result in program errors.


If I switch the modules and fail to add the analog module now in Slot 2 to the IO Config, I definitely get a CPU Fault. So it doesn't seem to be a Slot 2 issue as much as an issue specific to the module.


We've been pondering over this for some time (educational setting, but we WERE controls engineers in former lives), and really want to find out if anyone else ever noticed this, or could explain why a connected module, that is not configured, does not Fault the CPU.

 

[Ken Roach]

Welcome to the PLCTalk forum community !

My educated guess is that the program does not attempt to make a connection to (and verify the ID of) a module that is not in the I/O configuration.

When the IQ8OW6 is in Slot 1, with Slot 1 configured for an IF2OF2, then the controller attempts to make a connection, discovers the incorrect module ID, and faults the controller.

I don't know the details of the ribbon-cable "backplane" of the 1762 platform, but many similar systems use a two-conductor digital bus (I2C, SPI, etc) on each pair of pins, rather than an actual parallel bus for all of the devices (like CAN on the backplane of POINT I/O).