PLCs

It is optional. One way of doing it is with 1 rung:

1) examine on "start", or examine on "latch", endor, and examine on "stop", output "latch"

The other way is with 2 rungs:

1) examine on "start", set "latch"

2) examine off "stop", reset "latch"

Preference dictates the way to go. The preference here is to use the first option.

Can you explain a bit more carefully what you want to do?

Do you want to toggle an output alternately ON and OFF with successive pulses on one input? If not, then what?

As you've described it, one input could easily make an output turn on and off:

Input contacts made - output ON (not latched).

It is possible to do it with one input (input directly fed to output, input on = output on) or even no inputs (if using one of the PLC timer functions and automatic program start).

This is really basic stuff, even for a beginner. Have a look at the link below and on the internet for a basic overview of PLC functionality. http://en.wikipedia.org/wiki/Programmable_logic_controller

There are plenty of basic guides to help you (try a google search of "plc ladder logic tutorial").

The first three responses are all very good information. Here is one more link that may also help:

http://www.thelearningpit.com/

PLC tutorials, a glossary, a little bit of electronics... pretty good basic stuff.