Module A-151 (Quad Sequential Switch) is like an electronic four-position rotary switch.

It includes trigger and reset inputs, four in / outputs, and a common out / input. Each time a pulse is received at the trigger input socket, the common out / input is connected to the next in / output. After the fourth in / output, the next trigger makes it step back to the first again, and so on. A positive pulse at the reset input switches the out / input immediately back to the first in / output. Voltages in the range -8V...+8V at the O / I resp. I / O sockets can be processed by the module.

Four LED's indicate the active in / output (ie. the one that is connected to the out / input at any particular time).