Urgent question re. NPN S7 input card

Johnny T

Member
Join Date
Jul 2003
Location
Fife, Scotland
Posts
642
We are currently upgrading a 5TI Sequencer to a S7-313C PLC.

We've discovered today that the old PLC had inputs that were 'sunk'. ie. NPN.

We have already installed the new S7-PLC with PNP input cards and so, as you can imagine, we are right in the ... mire !

Does anyone know if Siemens do a NPN equivalent input card. This would allow us to just disconnect the wiring arms, do a slight wiring modification and be back on track...

Or is it going to be days of wiring hundreds of relays for us...

Please help urgently if you can, we're back on site tomorrow first thing and the customer wants a solution.. eek !!

Don't you just love PLC's ?!?!??!

:)
 
Hi Johnny!
What type of input devices are you currently using? Many solid-state sensors are switchable either through a physical switch on the device itself or in the wiring configuration. For instance, a prox or photo eye may have brown and blue conductors for supply voltage and a black AND white for the desired output (sinking or sourcing). Only one of which is actually used. I typically leave a few inches of cable available after installing such a component for just this sort of predicament. The installer may have merely cut off the unused output, but perhaps left enough for you to re-strip and connect for the sourcing configuration that your new plc is requiring. But then, that may also just be wishful thinking! Many manufacturers can make alot more money if they require you to specify the desired output (PNP or NPN). Bummer, huh?
 
I don't know why Siemens makes an input module that ONLY accept PNP inputs. Sure, that's all I ever use, but most other brands can be wired for either polarity based on what the COM (or Siemens "M") terminal is connected to.

I dug around the Siemens site (what a chore!) and found in THIS manual that the 6ES7321-1BH50-0AA0 specifically states for "SOURCING" inputs.

I hope this helps... :confused:

beerchug

-Eric
 
Did Terry Woods just post a reply with only two words?



Just funning Terry--no offense meant. Your contributions to this forum are far-reaching.
 
ColossalNut, thanks for your reply mate. The problem with re-wiring the sensors is that the factory makes paint and everything is caked in the stuff !! All the sensors are either Autumn Red or Sunshine Yellow !! ;-)) The junction boxes are buried under mountains of old paint too ! However, your suggestion is certainly an option...
Eric, you are a God. I struggled with Siemens Online Labyrinth myself and came up empty handed. We will look into getting some of those cards (Siemens helpdesk opens in 40 minutes.. and counting).
Terry.. thanks mate
:D
Rube, two words are better than none matey but thanks for giving the lad an ear bashing! (only messin' Terry, cheers mate)
:)

Fingers crossed for the rest of the job...
 
I agree with Eric that polarity for all DC inputs should be
selectable by jumper (S/S on Mitsubishi PLCs). Combine that with
relay outputs and you are all set :)

If you want to make it cheap, quick and dirty, just add pullup resistors
for the inputs. You will still have to invert them in software (couple of XORs).

You can also consider converters.

You know what, outputs could be universal too. It's not big
deal to have both PNP and NPN transistor (or P and N channel MOSFET)
on the same output - both for PLCs and sensors.
Take a look at inverter and buffer circuits for digital chips
like 4000 series for example.
 
Ah, the joys of DC controls. Sourcing input devices feed sinking inputs, sinking inputs feed sourcing inputs...just to make it simple.

Back on topic, I'm not an expert on Siemens, but I'd be shocked if they didn't have a sourcing input module and would even be surprised if its wiring interface wasn't identical to your sinking modules'. You might have to rewire the commons, at worst. IOW, you can come in off the ledge, Johnny.

Remember, not even Allen-Bradley has a field-convertible DC input module in its most common I/O series: 1771, 1746, 1794, etc. You have to be careful to order the IB or IV type as requird.
 
I've got the sinking input cards on order now. They are being DHL'ed from Germany, should be here by Friday.. then a lovely weekend of writing software, tearing out hair, crashing bits of palletiser etc etc.. oh joy oh joy :D

A big thank you for all your help everyone.. you've really dug me out of the mire on this one and it is very much appreciated. If we ever organise a massive World Wide PLC folk type get-together then I'll be the one at the bar asking you all what you'd like to drink !

:)

Cheers

JT
 

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