Hi Kalle
Of course they can be written in ladder but that only print an FB - the ladder is not exposed until online and open the FB up.
I write a lot of sophisticated software for controlling power stations, complex switching systems etcetera, including high voltage.
The reason they are writing 'ladder only' into specifications is for the above reason.
May I add that I have no problem with ST, except it is hard to debug. Great for complex maths though.
FBs are an absolute pain - if you want to change one and download to the PLC due tyo a contact coming in the other way, you have to stop the PLC, download, start the PLC again.
I gave FBs away about a year or so ago and went back to ladder exclusively, except for complex maths.
At least when a customer rings with a problem I can help get them online with the PLC and explain the code in a way that they understand with ladder. Makes it easier for me to trouble shoot as well as they can view and tell me what is going on.
Some of my customers insist on all programming in FBs - I then quote them an extra amount to write the software in FBs as I know it is going to take me longer to wriote the software and commission it! They usually take the ladder option and finish up agreeing with me at the end of the day when I explain to them why I generally use ladder. Cost them less too.
The secret to writing good, efficient and fast ladder and very fast comissionig is a very large and useful function set. Not many PLCs have that these days and insist of heading in the direction of FBs.
Many will probably disagree with me but when I can write and commission a full generator control system in a couple of days, for example, and commission it in a day, they do like the cost savings over the guy that was there before and took a week.
By the way, indirect addressing in ladder is easy with my favourite brand of PLC - do not need STL at all for almost everything.