From page 45 of The DCS O Gauge Companion 2nd Edition:
• To toggle a selected Variable Channel between Fixed and Variable modes of operation, press soft key S5 (FXD)
• A message is displayed indicating that the channel is set to either Fixed or Variable mode
• To turn the DCS signal on or off for all channels of the selected TIU, press soft key S3 (AON) to turn the DCS signal on or press soft key S4 (AOF) to turn the DCS signal off
• When a TIU Variable Channel is set to Fixed mode, it will remain in Fixed mode until one of the following occurs:
The Variable Channel is toggled back to Variable mode using Soft Key S5
(FXD). It will remain in Variable mode until it is once again toggled to Fixed
mode using Soft Key S5 (FXD)
The TR button is pressed, the track associated with the Variable Channel is
selected and the thumbwheel is turned 1 click. The channel will be placed in
Variable mode until it is once again toggled to Fixed mode using Soft Key S5
(FXD) or until the DCS Remote is powered off and then back on again