"Transition-On" One-Shots

[kdcui]

I have a Transition On contact in some code for an old Honeywell controller, however the documentation doesn't specify it's characteristics too well...

  100    200

--| |----|^|-------(300)

So if 200 goes from low to high, and XIC 100 is low, obviously the scan will not energize 300.

Now some scans later 100 goes high, and 200 is still high...will 300 be energized?

 

[bernie_carlton]

Is it possible that 200 is the 'storage bit'? That would cause 300 to trigger only on the rising edge of 100.

 

[kdcui]

Hi Bernie.

I was just using 200 as a "for-instance" example. But the transition-on is just the instruction type and can be addressed to any binary address (internal bit, input, etc).

I'm used to ONS one-shot instructions from PLC5 / CLX platforms.

If I had this:

  

  200 

--|^|--------(300)

then 300 would energize for one scan (unless it was a latch) on the rising-edge of 200.

What I am not sure of is if the OFF to ON transition of 200 has to be made and evaluated on consecutive scans.

In my first example, the transition of 200 occurs BEFORE the rest of the run is evaluated since 100 is still false. So when 100 finally does go High, 200 has already been High for more than one scan, so I'm not sure if the one-shot is then evaluated or only if the opportunity has passed.

On PLC5 / CLX it is easier to tell because the ONS instruction is not tied to the condition of a bit, but to whether or not the preceding logic is true or false.

Hopefully am not making this more convoluted than it has to be...

EDIT:

In the manual it mentions it mimics the operation of a momentary push button, but does not explicitly state what happens if the rung is still false when the transition occurs. If it truely is mimicking a pushbutton then I would think that the rung would have to be true prior to the transition.

 

[JHarbin]

The IPC620 does not need a storage bit like A-B.

In your example, if 100 is OFF then the Transition ON will have no effect.

Think of it this way, if 200 was a normal contact and it turned ON while 100 was OFF and then turned back OFF while 100 was OFF, nothing would happen.

Per the documentation:

"contact is TRUE only during program scan immediately following OFF to ON change of state."

 

[kdcui]

Thanks J,

I guess I was misinterpreting the instruction description.

 

[TConnolly]

If its like the modicon -|^|- then its based strictly on the state of the addressed bit, not on the rung (network) state.

 

[bernie_carlton]

So it's similar to the Positive Differential Contact in AutomationDirect

 

[JHarbin]

So it's similar to the Positive Differential Contact in AutomationDirect

Not really. The AutomationDirect PD element is a Coil and requires a second line of logic to use.

The Transition ON (and its cousin Transition OFF) is simply a single element that is ON for one scan of the processor when its associated input (or register) turns ON (or turns OFF in the case of the Transition OFF). It doesn't care about any other logic that may be in the rung with it.

 

[milldrone]

Not really. The AutomationDirect PD element is a Coil and requires a second line of logic to use.

The Transition ON (and its cousin Transition OFF) is simply a single element that is ON for one scan of the processor when its associated input (or register) turns ON (or turns OFF in the case of the Transition OFF). It doesn't care about any other logic that may be in the rung with it.

I believe this is what Bernie is referring to. Available on the 05, 06, 250-(1), 260, 350, 450, processors.

 

[JHarbin]

I believe this is what Bernie is referring to. Available on the 05, 06, 250-(1), 260, 350, 450, processors.

You are correct - the newer AutomationDirect Processors utilize those elements and they are similar to the Honeywell IPC-620 Transition ON and Transition OFF.

I thought Bernie was talking about the PD (Positive Differential) element which is a Coil and was what had to be used in some of the older AutomationDirect Processors.

 

[bernie_carlton]

You are right, I should have been more clear.

And the STRPD (or positive differential as a contact) can trip people up. When used with an input device it works as expected. But when applied to a bit which can be changed by the program (output or internal bit) its only valid use is from the point in the scan when the bit is affected through the end of that scan. If it's use is not with an input ('X') device then the output PD coil is the way to go.