Compax 3S: Possible to modify "C3_EtherCAT_full_2ms_V0209.xml" to add Additional Inputs and Outputs?

Good morning Matt.  Apologies for the late response.  Typically, editing the XML file is not the preferred way to change I/O on an EtherCAT slave device because it is, as you have seen, rather esoteric.  The XML files are usually only touched by device manufacturers and EtherCAT master configuration software (e.g. Parker Automation Manager or Beckhoff TWinCAT).

If the configuration software you are using does not give you the option of modifying the I/O map, editing the Compax3's XML file may be a way to achieve your goal.  However, I would consider alternatives first.

Does your EtherCAT master device support SDOs?  Any object in the Compax3 that has an EtherCAT mapping can be written or read via SDO.  This is convenient for things like servo gains that are only changed once in a while.

The "Valid After" column in the Compax3's documentation describes what you need to do to make the drive "accept" the input.  For instance, some changes occur immediately.  If you write to the Control Word (0x6040), the new value takes effect immediately.  However, if you write to a value like a servo gain, the new value is held back until a "Validate Parameters" (VP) command is issued.  The drive does this so that multiple interdependent parameters can effectively be changed all at once.  This is especially important for servo gains where changing one gain may require another to be changed to preserve stability.

You can issue a VP command using object 0x2016.10.

If you want to save the values permanently, you need to disable the drive and issue a Write To Flash (WF) command, which takes about 2 seconds to execute.  You can do this with object 0x2017: