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This is from the instruction manual of a printer:

Wrap ferrite bead around USB, 10Base-T, LPT but not RS-232

As you can see, one is not supposed to wrap the supplied bead around the serial cable. What is the reason for this?

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    \$\begingroup\$ If they only supplied one, perhaps they just want to make sure you use it where it will do the most good and not waste it on a cable where it is not really needed. \$\endgroup\$ – mickeyf_supports_Monica Sep 15 '12 at 3:53
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The bead may have a higher reactance than resistance at low frequencies, which could cause things like undershoot spikes to increase rather than decrease. Most likely these spikes will cause the higher voltage 12V signal to go beyond acceptable limits for RS232 and possible damage the devices. The other connection are all at lower voltages.

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  • \$\begingroup\$ Why do people down vote the correct answer and up vote someone telling them how to go against the manufacturers recommendations and possibly destroy their equipment? Hook your lines up to a CRO, you will see under volt spike extends dramatically with the ferrite bead in place. \$\endgroup\$ – Myforwik Sep 17 '12 at 7:50
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I honestly can't think of a valid electrical reason not to use the ferrite on the RS-232 cable. The ferrite acts as a common-mode choke that prevents RF interference from travelling along the cable, primarily in its shield.

At first, I thought it might have something to do with the fact that USB and Ethernet use balanced signals and RS-232 uses unbalanced signals, but then they tell you to put the ferrite on the parallel printer cable, which also uses unbalanced signals, so that isn't consistent.

Also, the speed of signaling on both RS-232 and the printer port are about the same (on the order of 100 kHz max.), so that wouldn't explain the difference, either.

I'd go ahead and use the ferrite on the RS-232 cable, too.

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