hi. if u know the true logic for all the gates like AND,OR 
etc,. its easy to write a program by using that gates.i 
suggest u please the site www.abb.com this will give what 
ever information u want,.

Ladder Logic is simply an interpretation of OR, AND, NOR, 
XOR, XNOR, NAND functions. First you solve the problem 
using the above functions & then transform the symbols into 
those of ladder logic.

Each rung of ladder language typically has one coil at the 
far right. Some manufacturers may allow more than one 
output coil on a rung.

--( )-- a regular coil, true when its rung is true

--(\)-- a "not" coil, false when its rung is true

--[ ]-- A regular contact, true when its coil is true 
(normally false)

--[\]-- A "not" contact, false when its coil is true 
(normally true)

The "coil" (output of a rung) may represent a physical 
output which operates some device connected to the 
programmable controller, or may represent an internal 
storage bit for use elsewhere in the program.

Example:
S = X AND ( Y OR Z )

----[ ]---------|--[ ]--|------( )--
     X          |   Y   |       S
                |       |
                |--[ ]--|
                    Z