It may hay been because the ACC ports are only momentary and non latching. All of my sw ports are used. Correct me if that is incorrect.
The AIU ACC ports can be either momentary OR latching. For momentary action an AIU ACC relay is triggered only for as long as you press and hold down the "ACT" button on the DCS remote. For latching action, the AIU relay is triggered when you press the "ON" button...and stays triggered until you press the "OFF" button.
The AIU SW ports are only momentary and non-latching. There are two relays inside the AIU for each SW port. One relay fires for exactly 1/2 second when you press the "DIVERGE" button on the remote. The other relay fires for exactly 1/2 second when you press the "STRAIGHT" button. There have been OGR threads on how to add an external latching relay to an SW port so that the SW port becomes latching...for example to drive a Tortoise or similar "slow speed" switch machine that requires continuous power. And if you run out of AIU ACC ports...but have a few unused SW ports, the same method of an external latching relay could be used to effect ACC port functionality ... albeit you activate the external relay ON and OFF using the "DIVERGE" and "STRAIGHT" buttons which may be awkward! Or just buy another AIU and get 10 additional genuine ACC ports since a TIU can support 5 AIUs.