Figures 2-13 to 2-16 show the previous anti-tie down circuit in ISO symbols. Most people understand ISO symbols since they show valve function more clearly.

The circuit is at rest in Figure 2-13. PB1 and PB2 are not actuated, so there is no signal being sent to the directional valve. This schematic is what the machine supplier sends with his documentation for the machine.

Figure 2-14 shows the circuit when depressing only one palm button. Here, PB2 sends an air signal to one port of the and element. The and element does not send any output because it needs two signals. The air signal from PB2 does go through the or element and starts TIME ON DEL1 timing. If PB1 is not shifted within a short time, TIME ON DEL 1 times out. When TIME ON DEL1 times out, it sends a signal through NOT1 and closes NOT2. After TIME ON DEL1 closes NOT2, the signal from PB1 through the and element becomes blocked at NOT2. Either palm button gives the same results. This protects the operator because depressing both palm buttons is the only way to cycle the machine. Before a cycle is possible in this condition, the palm buttons must exhaust any air signal from the or element, resetting TIME ON DEL 1.

Depressing both palm buttons simultaneously gives the results shown in Figure 2-15. Either one of the signals to the or element starts TIME ON DEL 1but one of the signals to the and element passes through NOT2, on to the directional valve. The signal from NOT2 also pilots NOT1 closed, blocking the output from TIME ON DEL 1. As long as both buttons stay shifted, the cylinder extends and holds.

After TIME ON DEL 1times out, the circuit changes to the one shown in Figure 2-16. Output from TIME ON DEL 1L1 stops at NOT1 because the start signal from the and element is holding it closed. With this circuit the operator has to keep both hands on the palm buttons to make the cylinder extend.

The following circuits show other uses for these elements and how more complex circuits use other logic valves.