Check out TRI-PLC.COM
They have some low end PLC's that are inexpensive. Programing is with a 
ladder diagram that allows embedded Basic routines. You can download a 
trial version of the software. They have a tech support forum, in 
addition to telephone that is very good. On the forum check out the time 
it takes for tech support to respond.

As was said, PLC's can be expensive, but one could take a PIC and make a 
PLC, but that would be a lot of effort, where a PLC might be able to do 
it reliably and quick program/setup. Since I got into thePLC's 4 years 
ago, there have been advancements, in particular in the low end units.

Jinx wrote:
> I'm assessing a job, which I think a low-end 18F can do fairly simply. The
> logic is quite straight-forward. Inputs are two 4-20mA transducers, a mains
> relay, and two temperature sensors, with a real-time clock. Outputs are
> a couple of alarm lines, LCD, and a few pushbuttons to set parameters
>
> The scenario might be something like - If transducer1 is a certain value
> and temperature sensor2 is a certain value, then turn on the mains and
> measure temperature sensor1 after 1 minute to make sure the appliance
> (a chiller) is running. And various IF....THEN combos like that
>
> I was talking over the pricing structure with a couple of people and they
> both suggested that I base it at less than the cost of an 'industrial PLC'
> which would be needed. Their thought is that the lower (but not too low)
> price mentioned in the same breath as a PLC will make my PIC sound
> more attractive. Can't say I'm 100% about that but they won't shut up
> about PLCs !
>
> I'm a complete ignoramus in the PLC department
>
> http://en.wikipedia.org/wiki/Programmable_logic_controller
>
> To me, after reading that, it seems a PLC is way OTT for this job, and
> especially if you know how to program a micro yourself. This assessment
> may be for 300 units (optimistic "Everyone will want one" salesman !!)
>
> I guess the material costs, not including sensors, using a PIC may be around
> NZ$70 (US$35-$40), and software perhaps a little less (@ Q = 300), then
> there's markup on that
>
> Can anyone with practical experience of PLCs give me any advice as to
> whether the cost of a PLC system is even worth considering ? And, if so,
> how do prices compare ?
>
> TIA
>
>   
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