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Tx and Rx are required in Computer networking, used in Cat5 or Cat5e or Cat6 cable? What does +(plus) and -(minus) sign mean in cable pairs for RJ45

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The positive and negative connectors in the cabling mentioned, form a balanced pair for signals. They typically carry identical signals of opposite polarity, i.e. a HIGH bit may be +5 Volts on the + conductor, and -5 Volts on the - conductor. The corresponding LOW bit would then be -5 Volts on the + conductor, and +5 Volts on the - conductor.

Having such signals of opposite polarity in a twisted pair of wires helps in reducing susceptibility to common-mode noise / EMI. Because the voltage difference between the two conductors for any valid bit is twice the actual signaling voltage, this has the effect of doubling the strength and hence the detection of the signal at the receiving end.

Specific to the cabling / protocol involved, various signaling schemes may be used for transmitting the actual bits down the line, from the simple Gray Code, to more complex non-return-to-zero (NRZ) or other coding.

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    \$\begingroup\$ RS485 is a good example for this. It is a differential two wire bus with line A and B also often labled as: A: "-", "TxD-/RxD-", "inverting pin" and B: "+", "TxD+/RxD+", "non-inverting pin" \$\endgroup\$ – Rev1.0 Jan 10 '13 at 14:02
  • \$\begingroup\$ @Anindo, does it mean(Taking NRZ encoding) that if current flows from Tx+ to Tx- its is equivalent to a binary 1 and if it flows from Tx- to Tx+ it means 0? \$\endgroup\$ – sk1 Jan 10 '13 at 16:00
  • \$\begingroup\$ @sk1 Not sure what you mean, but NRZ encoding essentially means there is no quiescent "Zero Volts between Tx+ and TX-" state while the link is active: When a "1" is being transmitted, Tx+ will be (for instance) 5 Volts, and Tx- will be -5 Volts, and when a "0" is being transmitted, these voltages will be inverted. Other options such as NRZi or Manchester are also used, depending on protocol. \$\endgroup\$ – Anindo Ghosh Jan 10 '13 at 16:33
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    \$\begingroup\$ @sk1 --- it isn't really analogous to power wiring. Since the lines are balanced, you don't need to return current in a "neutral" line. Therefore you don't use any neutral line. You just have 2 signal lines carrying equal and opposite signals. This can result in ground differences between the two ends of the line, which are often dealt with by magnetic coupling (transformers) at one end or both. \$\endgroup\$ – The Photon Jan 10 '13 at 18:33
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    \$\begingroup\$ @sk1, that's exactly right. Current flows out one line and back on the other. But neither one is a "neutral" line. Both have varying voltage in order to cause the current to flow. It is correct that "current in one direction means 1 and in the other direction means 0" but we usually analyze these circuits in terms of voltage, not current. \$\endgroup\$ – The Photon Jan 11 '13 at 17:02

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