Negative input logic directive???

[ToolGuyFred]

Re: May I recommend...

Somehow, the thought of someone so familiar with the names of these "products" coupled with the pursuits indicated by your avatar makes me just a little

Rest assured that I never indulge while in the UK and my firearms remain securely under lock and key at home when I visit the Netherlands.

I don't drink and drive, either. One has to be responsible about one's politically incorrect passtimes...

 

[parky]

It was thought some years ago that using npn inputs i.e. +24v signal off & 0v signal on would reduce the chance of spikes usually positive on a grounded system.
This did appear to be the case on a system I once had the misfortune to find a problem.
The company had 3 identical machines, originally these were japanese origin with plc's using npn inputs.
One had been upgraded to a siemens 110 using pnp inputs, only the sensors had been changed to account for the new input type.
This machine would on a number of occassions do funny things, now the software was simple logic nothing fancy & was a direct reproduction of the original (they had even had symbols with the I/O numbers presumably to copy directly the code as they only had a dump of the code.
I could find no reason for the faults but going back to my TV days remembering that on the old 405 line system the video was inverted this reduced the white spot syndrome with noise in the transmission so I fitted some 6.8k resistors across all inputs to the ov, this cured the problem, now was this noise on the inputs that did not show up on npn or was it generated from the sensors however it occured it never happened again so who knows
By the way, before anybody replies with why not use a scope, I was on-site not expecting to have to use one as this was a plc control system with a "BUG".

 

[ToolGuyFred]

Hi Parky

NPN inputs with 0V grounded may well be more noise resistant but there could be another reason. I seem to recall that older Japanese PLCs had inbuilt noise filters which could be configured with the programming software. Sometime back in the late 80s/early 90s I used the feature on a welding machine running a Japanese brick PLC (can't remember if it was Omron or Mitsubishi). It slowed the input response but practically eliminated spike problems. I believe that Siemens inputs have always been unfiltered without the option described above.

In another vein, one place I worked used a lot of PCs (personal computers, not programmable controllers) and their early kit had no opto-isolation on their inputs. The 5V NPN sensors didn't seem to be any more noise prone than the 24V PNP sensors on the nearby PLC kit...

 

[BobB]

Just as a matter of interest, the current crop of Omrom CJ/CS PLCs (have not tried with the CP series) has adjustable input filters per card. You cannot set a filter on a single input, only the whole card. If I remeber correctly, the filter can be set virtually anywhere between 1 and 32 ms.

I have never used a linear power supply - OUCH price - only ever switchers. The ones I normally use are inherently isolated. Usually use Omron for 240VAC/24VDC but sometimes Meanwell. The new Meanwell ones have a 3 year warranty - Omron have 2 years. Do a lot of generators with the control system running on batteries all the time. Trickle chargers sit at 27.7 volts - a bit too high for my liking in front of PLC power supplies. Use Meanwell 24VDC/24VDC switchers in front of the PLC. Gives me my desired isolation, constant 24VDC output and will still step up to 24VDC from a battery level of about 19 volts. Very handy.

 

[DairyBoy]

Parky, "inverting the video": out-of-the-box/inspired/genius; I love solutions like that. Ever read "Sirens of Titan", Kurt Vonnegut? "Turn the ship upside down." Too cryptic? Sorry.