Allen Bradley is one of industry leaders. It is very popular in North America since
it's local company (they have office in Cleveland, OH).
This means that manuals and support are better than for (translated) products from overseas
such as Siemens or Mitsubishi for example. Allen Bradley is often considered much simpler
to use too but it's products are considerably more expencive than competing equivalent.
once you get familiar with ladder logic it is very easy to work (program) just about any plc,
although one might have to swallow pill or two during inital setup.
it is not uncommon (at least if you are new to product) that initial setup is done by
distributor etc. there are also other languages like structured text (looks like any high
level language on pc), function block diagram (looks like collection of TTL or CMOS gates),
instruction list (looks like assembler) etc. function blocks are popular in europe while ladder
is dominating north america. ladder is visual language resembling relay circuits.
i preffer ladder for most day to day applications although other languages have strengths too.
while in the past one would usually have no choice but to use only one language for particular
product, nowdays it is common to see variety of languages for one and same product (within same
programming enviroment). i hope things will get better in near future since this multi language
support is often poorly implemented. for example, there are stil rather few products
that will allow comfortable work or completing of a project (if at all) in just one language.
that's why bulk of code in most cases is still done in ladder (or fbd). if you want to get taste
of function block or ladder logic, download Zelio_2 software. Zelio is not real plc, it's so
called smart relay but it is ok to see what the language is and you can even simulate (simulation
is something you see rare on plcs these days).
Rockwell Automation (Allen Bradley) has some of the products for download too (RSLinxLite for sure
but maybe some time expiring student version of RSLogix too, I'm not quite sure).
You can get mitsubishi software from
www.meau.com and it should work for a while without licence
(GX Developer for PLC programming, GT Works for HMI programming etc.).
someone mentioned parallel execution of logic. although it looks that way, that's not true.
logic is simply scanned very fast. often whole program is scanned continuously for simplicity
but one can use common forms of branching and subroutines to change that if application requires it
(faster scan is important in high speed packaging for example).
scann time will depend on plc and program size but it is usually in 1-20ms range. this is rather slow
compared to microcontrollers for example but PLC is not microcontroller. AC inputs and outputs are still popular
in some applications (like when wiring is very long etc.) but it is largely moved to 24VDC. it is not
only lower voltage and safer to work with, it is standard regardless of where machine will be installed
(AC voltage and frequency are different from countriy to country).
although standard scan time or digital I/Os are slower than on average MCU, PLCs have hardware that
can take care of time critical or complex tasks (counting, positioning, communication etc.).
also plcs can have far more i/o than MCU (most PLCs i work with, support up to some 8k inputs and 8k outputs
and this can be further expanded using fieldbus networks etc.) but this doesn't mean that my projects
get this large. the biggest one i worked on was using DCS (big brother of PLC) and it had only about 4k I/O,
(most ware anlogue - there was some 1200 PID loops among other logic). most projects i work on these days
are limited to some 150-300 I/O. another significant feature of PLCs is ability to change running program
(modify existing, not download new one) without stopping process. we use this all the time since
many processes cannot be simply shut down (you don't want to stop line in car assembly plant just because
you have to rewrite some logic. you simply keep adding/deleting/editing and testing logic
while equipment is running). major changes however, are better done offline (in my work field this
may mean working on a weekend or even at night).