What are the advantages and disadvantages of PLC (Programmable Logic Controls) over traditional relay wiring controls?

Advantages...

-Troubleshooting is Easier and  Faster..

-Easy to develop Programs by offline simulation

-Less amount of Space Needed

-Less Maintenance

-Easy to Monitor Inputs and Outputs by HMI devices....and from PC's

Disadvantages.

-Cost of PLC is high

-PLC'S are designed by semiconductors, which are depends on thermal characteristics

1) As PLC's have solid state switches, they can provide unlimited switching cycles so they can be used to create PWM signals.  

2) PLC's are re-programable

3) A very complex logic can be created without the need of complex wiring.

4) Multiple devices are embedded in a single PLC like Timers, Memory Shells etc. and also multiple sensors can be attached like temperature, humidity, motion, speed etc.

5) the function of switches connected to PLC's can be changed as they are re-programmable, also the connection between a switch and a load can be changed in software. If all the lights of a house and all the switches were PLC controlled (E.I.B. systems) then you can decide witch switch shall control witch light without the need of re-wiring. 

and many more.....

When designing a logic diagram using PLC or programming logic devices it has many advantages over the traditional wired logic some of the advantages besides a fast response time:

1- complex logic can be achieved using the same PLC unit.

2- means of controlling many incoming and outgoing signals, could be digital or analog using one Unit or exapnsions of the PLC unit.

3- reduce cost and errors as mean of service wise.

4-reduce space occupied by the traditional wired logic ( could be critical in some products).

5- some PLCs can be connected to a human Machine interface as an LCD which could show a lot of info the end user would get use of.

some of the disadvantages of a PLC i came across :

1- You need a Programmer (know-how) skill.

2-debugging the PLC sometimes consumes alot of time.

3- Working Temperatures is an important factor, as some of PLCs work with max. operating temperature very less than the gulf summer ambient temperature - and how if it was embedded in a control box with no ventilation.