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I have read that

RS-422a cable uses a 20 mA current loop instead of voltage levels. This makes the systems more immune to electrical noise, so the cable can be up to 3000 feet long.

I do not understand why having a current loop will provide more protection against noise. Can someone please explain? Also, does that mean RS422a uses current to turn ON/OFF instead of voltages?

KMC
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    I'm not an expert on this, but I think the fact that RS422 is differential (balanced) and RS232 isn't may make RS422 more noise immune than RS232. – Saad Feb 24 '12 at 07:37
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    Current loop, RS-232, RS-422 are tree (3) quite different communication methods. Also, it would help if you wrote more about your hardware, installation, environment it operates in. – Nick Alexeev Feb 24 '12 at 07:44
  • @Saad: I believe RS423 and up are differential, but not RS422.. – KMC Feb 24 '12 at 08:42
  • @NickAlexeev: there is no hardware. I am just trying to understand a conceptual introduction of RS232 and RS422 on a textbook. – KMC Feb 24 '12 at 08:43
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    @KMC http://en.wikipedia.org/wiki/RS-422 Suggests that RS422 is differential. – Saad Feb 24 '12 at 11:00
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    RS422 and RS485 are differential, RS232 is not. All three are voltage-based. The differential voltage is much more noise immune, due to noise affecting both lines generally equally, which cancels out. – darron Feb 24 '12 at 17:05
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    @darron: IMHO Your comment really should be submitted as an answer since it explains OP actual question well. Comments tends to get drowned away. – Imbrondir Feb 25 '12 at 21:02

2 Answers2

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note: answer is about current loops, not RS422A

Current loops have a number of advantages over voltage lines. A 4/20mA loop, which uses 4mA and 20mA to represent two logical levels, allows for instance to detect cable breaks, since in case of an interrupted cable the current will be zero. Another advantage is its noise immunity, which is important in industrial environments with high power machines. The reason for the noise immunity is the low impedance of the line; a 250\$\Omega\$ resistor is enough to create a 5V level on the receiving side.
Voltage lines on the other hand are high impedance, which results in low-power. (Here the current loop has the disadvantage.) High impedance makes the line noise sensitive: since the noise energy has nowhere to go (due to the high impedance) it can easily build up a relatively high voltage. The low impedance of the current loop ensures that any injected noise is immediately drained to the load, so it can't build up high voltages which would disturb the loop's current.

edit
The high power requirements are the main reason current loops are only used in industrial environments. Low-power voltage lines typically use differential signalling to increase noise immunity. Explained in this answer.

edit 2 (re Russell's comment)
RS422 is differential (thanks Russell), multi-drop, and improves on RS232's speed. I wasn't sure about the differential thing, but it's normally not used in current loop systems. Anyway, RS422 is voltage driven.

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stevenvh
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  • Very clear and complete! – clabacchio Feb 24 '12 at 08:51
  • RS422 is differential signalling. – Russell McMahon Feb 24 '12 at 11:48
  • @Russell - Well, I wasn't quite sure, so I don't mention RS422, and only talk about current loops instead. I know RS485 is differential. – stevenvh Feb 24 '12 at 11:52
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    Also, switching currents can in general be done faster than switching voltages (parasitic inductances tend to be lower than parasitic capacitances, at least relative to the voltage/current levels used). – Jason S Feb 24 '12 at 14:31
  • @stevenvh: RS485 is (electrically) almost the same as RS422. RS422 has differential pairs for both TX and RX, so it's full duplex. RS485 ties those pairs together and adds driver enables so it becomes half-duplex. The driver enables make RS485 a real pain, relatively. – darron Feb 24 '12 at 17:02
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I don't know if RS422 uses current on/off for signaling, but if it does that should in theory provide better signaling. Voltage can vary due to resistance in cable due to for instance length, imperfections (subtractive) or induced/electromagnetic noise (additive).

Current on the other hand is constant across the entire line. Transmitter voltage can be adjusted according to higher or lower cable impedance.

Imbrondir
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