# What happens when there's no specific input variable on a logic diagram using a JK flip flop?

I'm trying to do some homework involving this circuit diagram: But I'm confused as to what the input would be for building a state table. Would I have to use the states of the flip flops as the input variables?

• your instinct is correct - assign labels to the FF outputs, e.g., Q1 and Q2. to form a table, on the left side, list the the four possible states of Q1, Q2; on the right side, write the values that Q1 and Q2 will assume after the next clock pulse. The table will then contain a complete description of the state machine. – JustJeff May 26 '11 at 0:53
• thanks. when there's a diagram like this, would you assume the output is the Q' on the right-hand JK flipflop? On the other one's i've seen it usually specifically shows the output. I need to make a state diagram, so would I just omit the intput/output value that's usually included on the paths between the nodes? – Marty May 26 '11 at 5:44
• @JustJeff - This should be an answer. – stevenvh May 26 '11 at 7:32
• @stevenvh - all righty then – JustJeff May 26 '11 at 10:33

Your instinct is correct - assign labels to the FF outputs, e.g., Q1 and Q2. To form a state table, on the left side, list the the four possible states of Q1, Q2; on the right side, write the values that Q1 and Q2 will assume after the next clock pulse. The table will then contain a complete description of the state machine.

So I was bored, and Falstad Sim is Awesome.

This should get you started. The Falstad Circuit Simulator applet is great for doing little logic proof-of-concepts, or other similar idea verification tasks. Like any circuit simulator, it will have situations where it does not behave exactly like the real-world circuit would, but with logic, it is certainly close enough.

When we are working with the flip flops the state of flipflop is very important so we have to consider the state of flip flop only.

the another reason is that we dont know what is the previous state of the flip flop so we have to consider all the possible states (0 and 1)to know the exact behaviour of the circut.