I assume you have checked this page:
There will be a bit of worked involved. Firstly, the hardware part:
- the pi used 3.3V signals.
- the PLC used Rs232.
= this means you will need to have circuits in between, integrated circuits doing this saves a lot of work, sometimes called line drivers. Try googling "3.3v rs232 line driver"
Next you need to setup the low level protocol. That includes setting baud rates and parity and such. This is more or less standard functions in linux. You could do it from a command line or from a program in (among other choices) python or c.
Next you will need to find out how the "programmer" works and what it expects to see. It could be simple, or it could turn out as a major programming effort.
If you are really lucky, the programmer expects to connect to a tty port on the network -- this is more or less already part of linux on the raspberry, you simply need to sort of "connect the dots".
I have done similar things in other environments, nothing with Allen Bradly though, cannot really help with the specifics.