Always On bit or something else

Today i came across a PLC unit from texas instruments, can't remember the part number of the CPU, but the software it was running is TI-Soft.

So somewhere in the Logic i saw logic written in this manner

c121 c121
----|--] [--|---------------------( )
| branch |
| branch |
| c121 |
|--] / [--|

the lines coming down represent a branch, on top the c121 in NO contact and on the bottom c121 is a NC contact

to me this looks like an always on logic where c121 is always on once the PLC goes into run mode.

My question is, is their any other instance where this kind of logic can be used for something else?

Also who's bright idea was it to make the msb (most significant bit, bit 0 and LSB bit, bit 15) ??

In Tisoft that always high bit was used in the rung before what was called a 'box'
The box contained a mathematical instruction, but Tisoft did not allow a direct connection to the left rail. This always high bit was used to make the box work unconditionally.

If you use the ladder tag you can draw a ladder using text. If you go to the advanced post window and look at the top of the text box where you type your post you will see a little icon that looks like a N.O. contact. Click that and it will insert the ladder tags. Now draw your ladder between them. A word of caution, if you hit preview or if you edit the post then its not going to look right afterward.

Here is your ladder between the ladder tags.

      C121                       C121

----+--] [--+---------------------( )-

    |       |

    |       |

    | C121  |

    +--]\[--+

You are correct, C121 will be always on. It is easier to just do an unconditional rung
-----------------------( )--
however sometimes on some platforms a bonehead somewhere decided that such a thing shouldn't be allowed and wrote the compiler to reject the rung. In those cases you have to have some kind of logic that will be always true to turn on the bit.


As for which bit is significant, you have just stumbled upon a big endian. Motorola processors are big endians while Intel processors are little endians.

thanks guys appreciate

TConnolly said:

As for which bit is significant, you have just stumbled upon a big endian. Motorola processors are big endians while Intel processors are little endians.

Click to expand...

Strictly speaking "endianness" refers to byte order, not bit order, which the wiki article you linked to explains precisely.

That doesn't answer dbh6's question - "whose bright idea..."... and i don't know the answer.

It has always been common practice in most numbering systems that the right-hand "character" represents the value of the lowest significance, so reading a number left to right goes from highest to lowest significance.

TI (and a couple others) went astray with their bit significance, and of course it also asks the question "how do i write it down" (as in left to right) "... MSB to LSB, or LSB to MSB?"

It would certainly be most strange in the decimal numbering system to quote pi as 456295141.3

Incidentally, the TI PLCs numbered chassis slots from right to left as well, with the processor in the right most slot and the highest numbered slot on the left end.

TConnolly said:

Incidentally, the TI PLCs numbered chassis slots from right to left as well, with the processor in the right most slot and the highest numbered slot on the left end.

Click to expand...

Too many mirrors at TI, then. They just went crazy seeing things backards (AAF), !!

I've just spotted the conundrum....

In the hardware world, we generally go left to right - terminal numbers, library indexes, slot numbers, dorm lockers etc. etc.

But in the maths world, it's the opposite, because we want the higher significance on the left.

Perhaps TI got it right, and we're all living in a fantasy........

TConnolly said:

Incidentally, the TI PLCs numbered chassis slots from right to left as well, with the processor in the right most slot and the highest numbered slot on the left end.

Click to expand...

I think this was only the 300 series, which was actually a Koyo Kostoc aka GE Series 1 aka DL305, which is still available.
In the 500 series at least the 525/535/545 had the CPU on the left.
I cant remember the TI530, a long long time since I worked on one of those, although my curiosity is aroused so I will check it out too.