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GEOFRA
March 9th, 2004, 11:25 AM
I have to change an old PLC on a very old pallet stacker which has approximately 70i/p and 70 o/p.
I am using SIEMENS PLC and as I am more experienced in S7-200 I prefer to do it with these instead of S7-300.
The problem is that the digital i/p of S7-200 accept DC volts and all the inputs on the machine are 24VAC.I have tested it in my workshop and the PLC operates ok with 24VAC on its inputs.Actually the i/p of SIEMENS are opto and they have two diodes which in this case rectify the AC without any problem.
I wonder if I can rely on this or if its a great risk of me visiting this factory all the time for problems caused by the PLC.
Does anybody have experience with such a case?

kennyb
March 9th, 2004, 01:51 PM
am new guy here but i think an s-7 a much better prospect for this job. i've only been messing about with siemens for a short time but after the initial shock of step 7 am well into it. (if only i had more time to write) anyway, something like an s7-315 2 dp isn't too OTT and allows good scope for later upgrade/expansion. thats what i'm trying to convince the dinosaurs here anyway (s5,melsecfx, 90% 0f factory no plc, dcs etc.!!) - while i'm here, does anyone have any ideas on a routine to measure the time (in ms) between pulses of a PEC on a DI. it is to allow me to calculate continually PPM values. all my ideas so far are no good.
cheers

kennyb
March 9th, 2004, 02:11 PM
I should have been a bit more detailed. I would definitely not take the chance with AC inputs on a DC I/O module. I think you would end up being a regular visitor to that job.The I/P configuration maybe allows it to work but I would not take it for granted unless there's a label somewhere which says"AC/DC" inputs.

jvdcande
March 10th, 2004, 01:00 AM
Shouldn't be too hard to replace the AC power supply with a DC one. Or even add a DC one for the inputs only. That way you could do without eventually replacing AC components on the output side. Just be sure to use a PLC with relay outputs in that case.

An other solution is to put relays between the sensors and the inputs and use a small PS to only power the inputs.

Kind regards,

azlee
March 10th, 2004, 03:57 AM
I have Gppw in japanes version so I hope that some one can help me to download Gppw in english version

wadehassler
March 11th, 2004, 12:09 PM
Don't do it. Unless the AC-inputs have filtering as well as diodes, the 50/60Hz input will *eventually* (in milliseconds or years) be read as off when it's plainly on.

GEOFRA
March 12th, 2004, 12:52 AM
Thank you all for your reply.I finally decided to use S7-300 series which offer 24VAC input modules.