Reverse output pulse sequence of Johnson walking-ring counter ?

[Steve Parus]

I'd like to reverse the output pulse sequence order of a 6 output
Johnson (walking-ring) counter constructed from three 7474 D flipflops
which are wired as Q1 to D2, Q2 to D3, Qbar3 to D1. This produces six
outputs spaced 60 degrees apart with a sequence of Q1, Q2, Q3, Qbar1,
Qbar2, Qbar3. I'd like to be able to reverse this output sequence
with a switch. A 6PDT switch could be used to simply change what
flipflop output is connected to my actual output. I was hoping though
that there is a way to use a three pole switch instead (easier to come
by in a toggle style) and somehow change the connections of the
various D inputs and Q or Qbar outputs.

Steve

 

[Dana Raymond, a minor God]

It seems to me that one important question must be answered before a
solution can be proposed.

When the switch is activated what happens to the outputs other than the
sequencing reversal? Is the switch action asychronous (changes states
immediately and possibly noisily)? Or should the sequence change at some
specific point in time or state?

You could replace the FF sequencer with a simple PLD (16R8 or whatever) and
wire one of the inputs up to a SPST switch. The PLD would be programmed to
give the proper sequence for both switch on and switch off.

Dana Raymond

 

[Steve Parus]

Not critical, it could change immediately or at any point in the
cycle..

 

[Steve Parus]

Thanks for the tip. What works in general for Forward is D connects
to the previous Q except for the first flip-flop whose D connects to
the last flip-flop's not-Q. For Reverse, D connects to the next Q
except for the last flip-flop whose D connects to the first
flip-flop's not-Q. So for four flip-flops only a 4PDT switch is
required to control eight outputs.