Siemens SM321 Digital input maximum cable length?

Hello guys,

I have an issue about maximum cable length calculation of a digital input on Siemens SM321 card.

by calculation and according to tech data (Link at the bottom) :

Max cable resistance should be (24 VDC - 13 VDC) / 0.0042 = 2619 Ohms, 13 V and 4.2 mA being the minimum voltage and current @on state. If one consider some connection resistance (allowance) we can safely assume 2560 Ohms as maximum cable resistance. By using a 1.5 mm2 cable of 12.8 Ohms/km resistance, it turns out the maximum distance should be 2560/(2x12.8) = 100 Km.

In the other hand, by looking at the tech data , Siemens recommends max cable length of 1 Km only.

Could anyone explain please why this discrepancy (huge difference)

Thank you in advance

Link to Tech data : https://support.industry.siemens.com/cs/pd/46945?pdti=td&dl=en&pnid=13763&lc=en-WW

I am guessing that dangerous differences in voltage potentials are much more probable for very long distances.
And lightning strike are also much more probable, because of the distance and because if you have 1km+ then you are probably laying the cable outside or betweeen buildings.

Thanks Jesper for your contribution.

I see no evidence from Siemens data that they consider lightning or difference in voltage potentials, besides this is digital signal and supposedly in a well earthed installation, actually a have a distance of 3Km in my case and I have doubts.

I understand that it will be difference between theoretical calculation and actual practice but not that much 100 km to only 1 km !!.

Either my calculations are false or there is some other thing to consider.

azetou said:

besides this is digital signal and supposedly in a well earthed installation,

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The internal electronics in the digital input module operate at 5V TTL levels. They are extremely sensitive to lightning strike. Even if the metal structure of buildings is earthed, high voltages can find its way into sensors and cables as the lightning current tries to find the easiest way to ground.
This why there is such a thing as SPDs (surge protection devices).

edit: If you have 3 km distances, then voltage differences, due to lightning or other reasons will be an issue.
Definitely you must have an equipotential connection - not just connect everything to the nearest ground.
And SPDs may also be necessary.

If there is any possibility of stray voltage on a digital signal line it is best to install an interposing relay between the wire and the PLC. It is much better to toast a $10 relay than to damage a PLC input card.

azetou said:

Either my calculations are false or there is some other thing to consider.

Click to expand...

I don't know anything about this spec specifically, but something else to consider is that not all the spec numbers have the same context. They could be the best case max (you definitely won't be able to go past 1km), the worst case max (1km should definitely work, depending on conditions you could go further), or just whatever was convenient to test (the longest cable the intern in the test lab could find was 1km).

I would think that the maximum cable length specified will mean the cable capacitance will allow the input to meet it's reaction time. If your input is changing very slowly (say once every ten seconds) then 3km of cable will be ok.

If you have many IO to connect that cover such a distance, it would be worth considering remote IO of some form if the IO is grouped in the same physical location.

Pulling 3km of cable isn't fun. And when the cable fails because it was direct buried and backfilled with sharp rocks because good bedding is expensive... it'll be even less fun.

L D[AR2 said:

I would think that the maximum cable length specified will mean the cable capacitance will allow the input to meet it's reaction time. If your input is changing very slowly (say once every ten seconds) then 3km of cable will be ok.

Click to expand...

this is direct current so capacitance of cable will be of almost no effect, besides i will be connecting just a hand switch to send an alarm. I hope that it will work for 3KM of cable.

To saffa, I will be using a spare pair of cable that is already intalled.

Thanks for your contributions but the question remains why the big difference in cable length?.