Sequential Function Chart

[Paulus]

Whew...
I guess that like most PLC issues, there are plenty of pros and cons depending on ones experiences / points of view.

However, judging by the responses so far, it seems that there's still plenty of life in SFC as a tool.
I must admit, that my only previous experience was having a look at some dodgy old Modicon software, which had SFC as a 'bolt-on' rather than as an intrinsic part of the programming package - I don't know whether the same is true for other manufacturers? (Although obviously from Rons' response, A-B do at least with one of their packages).

Any of you SFCers out there have ideas about what is good or bad about particular implementations of SFC (i.e. what makes it easy to use / easy to understand / debug etc - not forgetting what makes it really cr*p)?

 

[mordred]

as far as ones and zeros are concerned I'll stop at hex. its bad enough when building device drivers for self developed hardware.

Granted Masm is'nt a particularly complicated compiler.
any tool is useful and mostly depends on individual flavor. I can see this programming language being useful. I however don't think it will replace the traditional ladder logic. However not everyone enjoys ladder logic either.

 

[Gerry]

I've tried the A-B variety on a test bed processor. I like the idea of it but, since online editing isn't possible, I can't use it.

I've argued this point with various people over the years. Basically, I don't accept it as valid.

The SFC is a "high-level" organisational tool that should be finalised early in the design process. A requirement to edit an SFC on-line indicates a lack of understanding of the target system's functional requirements. In the PLC5, actions and transitions are ladder files that can be edited on-line.

That said, you can edit SFC's on-line in the ControlLogix processors as of version 13. SoftLogix 5 also permits on-line edits to SFC's.

 

[RMA]

For those of you who couldn't picture what I meant when I talked about hammering machine code into a PDP11 with its toggle keys, here's a screen dump of a page I fell over by accident while looking for something else.

The toggle keys have a very light action, so to tip a key up for a "1" you didn't need to turn your hand over, instead you just hit the keys with your fingernails. The colour has suffered a bit in the picture, but in fact the darker keys were a violent lilac which contrasted very well with the red of the other keys, so your groups of three keys (this was an octal machine, remember), were clearly defined. In addition, if you scale the 19" rack mounting, you'll find out that the keys are between 0.6" - 0.7" across - almost exactly finger diameter, unless your name's Mike Tyson. You also notice the keys taper towards the front which helps to make sure that you don't catch an adjacent key.

So now all you have to do is use three fingers on each hand, the "1"s go in on the "UP" stroke, the "0"s on the "DOWN" stroke and you've got that register programmed in half the time it would take you to type it on your PC keyboard, never mind on the silly little Decimal key-pads bawling of later PDP11s!