stuck on a simple bit issue...brain fog as per usual

Skidood

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Join Date
Oct 2016
Location
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Hi Group

See the photo.
Will this work? I previously had the "contactor outputs used as inputs" tied directly to the IO DIG OUT coils, and what happened was, Valve 2 did not activate when Contactor 1 was ON.
Then I added the extra bits as shown but I think I'm still breaking a rule and haven't really changed anything.

i can't figure out how to do this any differently....help!

BTW..It might be evident to you all, but what I want is Valves 1 and 2 both energized when Pump 1 is ON, and only Valve 2 energized when Pump 2 is ON. Pump 1 and Pump 2 never run at the same time, its either one or the other.
Thank you so much...

cccc.png
 
Last edited:
[re-formatted:]


  1. Valves 1 and 2 both energized when Pump 1 is ON, and
  2. only Valve 2 energized when Pump 2 is ON.
  3. Pump 1 and Pump 2 never run at the same time, its either one or the other.

So stated another way:

  • Valve 1 is on when Pump 1 is on
    • Valve 1 is unaffected by Pump 2's On/Off state
  • Valve 2 is on when
    • EITHER Pump 1 is on
    • OR Pump 2 is on.


[N.B. Fixed the "Canonical" comments; thank you kamenges!]


Code:
     Pump1On   Valve1
-------] [------( )---      ### Canonical [C := A]



     Pump1On    Valve2
---+---] [---+----( )---    ### Canonical [D := A or B]
   |         |
   | Pump2On |
   +---] [---+
N'cest pas?


Btw, I would still code summat for a violation of case 3 e.g. an alarm that summat (sensor?) is amiss.
 
Last edited:
Originally posted by drbitboy:

Pump1On Valve1
-------] [------( )--- ### Canonical [if A then C]



Pump1On Valve2
---+---] [---+----( )--- ### Canonical [if A or B then D]
| |
| Pump2On |
+---] [---+

I am going to get a little semantic here but, since words really do matter, I'll risk it.

Either a very important precondition was not stated in the diagram above or the canonical descriptions are incomplete. In that description form it should be:

if A then C
else NOT C

This is something that drives plc users that come from text based languages a little nutty sometimes. The standard coil implementation in all plcs that I know of contains an inferred "else NOT" function.

Another way of thinking of this is
"C = A" in the first case and
"C = (A OR B)" in the second.

Keith
 
I am going to get a little semantic here but, since words really do matter, I'll risk it.

Either a very important precondition was not stated in the diagram above or the canonical descriptions are incomplete. In that description form it should be:

if A then C
else NOT C

This is something that drives plc users that come from text based languages a little nutty sometimes. The standard coil implementation in all plcs that I know of contains an inferred "else NOT" function.

Another way of thinking of this is
"C = A" in the first case and
"C = (A OR B)" in the second.

Keith




d'Oh, thank you, yes, I mis-posted, my mistake, this is much better., I will fix it.


Did I at least get the ladder right? ;)
 
Last edited:

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