Digital input maximum distance

[redeemer 0]

Hi, I was wondering, what is the maximum distance that a digital signal can run. This sidnal does not change much (commands for starting and stopping equipment). The signal should of course does not give a wrong position (hi instead of low, and vice versa). Probably this would differ according to the environment, the place I have in mind is a plant floor with some motors nearby. Does anyone have some real life experience with the maximum safe distance? Thanks

[JimRowell 1]

You won't generally have any problems in a normal factory setup. It depends on too many things to state a distance catagorically. Use only DC. Low current AC can bring problems with cable capacitance. Also, don't use too small of a time constant when filtering for jitter on plc inputs. Increasing the current flow helps with any method (you can add a resistor across the input and ground). A higher current tends to swamp out any effects of induced noise. Stay away from sources of noise like transformers, inverters, power cables, flourescent lights, etc. I havn't personally run into any situation where line length was a problem except when using AC (or relays). A few years ago, I was part of a team installing a medium size plant using 24vac. No one believed me when I expressed doubts about the wire length. Well, on our first startup, only a handful out of several hundred plc and relay inputs worked at all. We had to add resistors all over the place in order to solve it. Cable capacitance was the problem. Voltage drop is not usually a problem with the low currents used by plc's. And noise doesn't seem to be as big an issue as you might think. Maybe I've just been lucky. Jim Rowell

[Steve Bailey 118]

Jim has covered the issues. It's my personal experience that before signal loss due to the length of the conductor becomes an issue, the cost of that wire and the conduit in which it runs makes it worthwhile to move to a remote I/O scheme like DeviceNet, Profibus, Ethernet, etc.

[redeemer 0]

What is the trouble with relays here? Fadi Mansour

[JimRowell 1]

Well, relays can be super-low-current like plc inputs or they can be huge things that need lots of current. I've seen many, many cases were people ran long lines to relays and find that the voltage drop won't allow the relay to pull in. It just depends on the size of the relay or contactor as well as the "sensitivity" (efficiency and strength) of the coil whether it falls into the same class as plc inputs or not. Very few are as low as plc inputs. On the other hand, the lower the current of the relay coil, the more prone it will be to the other problems such as noise and induced current and cable capacitance (just like plc inputs). Jim Rowell