oops, pnp npn help please.

I have an old AB plc (defunkt and beyond repair) to replace in a panel.

I am changing it for a different type.

Here is the oops.

The model I have, is either all sink input or all source input, the old AB had a mixture of both. (didnt see that when I was quoting)

I have decided that it will be all negative inputs because of the npn prox's that pulse at high speed.

I know there is a way, But I dont know how........ How to change the + inputs to - inputs without using relays.. or use all + inputs and change the prox's - inputs to +

http://www.plcs.net/dcforum/DCForumID1/1974.html

That link explains using pull up or pull down resistors to covert npn to pnp/pnp to npn. Note the logic has to be reversed too.

If you want to look at solid state relays this may help:
http://www.crydom.com/userResources/productFamilies/38/crydom_dc60.pdf

http://www.crydom.com/default.aspx

"npn prox that pulse at high speed" and "using relays to change polarity" only makes sense if your relay is fast enough. all proxy sensors i've seen have switching freqency in the range 500-5000Hz
which is faster than standard PLC input can see anyway.
electromechanical relay will definitelly not be able to help you in this case. you better look for solid state solution.

Phoenix Contact makes a small module to solve your problem. Partnumber 2964319 DEK-TR/INV.
This inverter module inverts the signals of ground switching n-p-n transistor outputs into plus switching p-n-p outputs, and vice versa.
My sales guy calls it the 'OOPS' module.

I think you should go read this Interesting Article on Industrial Control Safety





http://www.mrplc.com/kb/index.php?page=index_v2&id=44&c=38

Thank you for your kind answers.

however, I am going to use negative inputs because of the npn prox's (8 of them that pulse about 7 times a second (they are for counting sewing actions on a sewing machine)
The rest of the inputs are at the moment positive inputs (from positional pnp prox's and buttons and limit switches)

I can use interface relays for these to invert the signals and there is no problem with the logic as I am writing the program from scratch.

I just wanted to know an easy way instead - to use pull-up or down resistors and how to wire them.

after reading the replies, I am still not clear on how to use or where to put the resistors from the 24V input signal to change it to a 0V or no signal input.

thankyou

When you put the resistors in, don't forget to calculate the wattage required. I forgot this step in the past and ended up frying a couple 1/4 watt resistors. A good understanding of Ohms law helps:

http://www.patchn.com/electron.htm

Think of 1 as the plc input

When switch S1 is open (off), pin 1 is tied to Vcc through the resistor. Since pin1 is a high impedance input, a voltage meter or logic probe placed on pin 1 will show Vcc (+24v) if connected to pin 1.

When switch S1 is closed (on), pin 1 has a direct connection to GND, which takes it to the low state. The pin1 side of R1 also has a direct connection to ground. Current will flow from Vcc, through R1, and to ground. It isn't considered a short, however, because R1 will limit the amount of current that can flow to a very small amount. In fact, you can compute this using Ohms law.

From Ron's example above,

Power = I^2 * R = (.0024)^2 * 10000 = .0576 W
In this case, a 1/4 Watt resistor is fine.

When I tried this the first time, all I had was a 640 ohm resistor.

I = V/R = 24/640 = .0375A
P = I^2 * R = (.0375)^2 * 640 = 0.9 W
My 1/4 watt resistor turned into toast!!!