A vital addition to the Doepfer A-155 sequencer, the A-154 adds playback mode changes (forward, backward, pendulum, and random) as well as one shot modes.
It also adds what's called "pulse width" and this isn't what you might think - This actually adjusts the length of the gate/trigger output from the sequencer, and CV control over everything.
The sequence controller replaces some of the functions of the A-155, but there's the ability to turn these new controls off and resume the A-155 as a stand alone sequencer as if you hadn't connected the A-154. It also offers the ability to switch between one and two A-155 modules in a single sequence.
The switch that's labeled 8-16 gives one the impression that it can turn any sequence with a single A-155 into a 16 step sequencer, but this isn't the case. In order to make use of this feature you need two A-155 modules and the switch allows you to switch between them.
The charm of the A-155 by itself is without question, and yet the A-154 completes the sequencer. Add a sequential switch and a quantizer and you're really getting somewhere!