RJ45 and other modular cables

About the Author: Lammert Bies is a dad, husband and polyglot. He is developing embedded systems since the eighties. Used machine learning before it had a name. Specializes in interconnecting computers, robots and humans. Was a Google Mapmaker Advocate and speaker on several international Google conferences from 2011 until the plug was pulled on Mapmaker in 2017. Bughunter with Google. Currently spreading artificial intelligence to the wildest locations in production environments. He never stops learning.

• RJ45 pin numbering
• Straight through RJ45 network cable
• Cross over RJ45 network cable
• Common wiring schemes
• DEC MMJ cables and adapters
• Yost RS232 on RJ45 wiring
• Unitronics RJ11 wiring

RJ45 pin numbering

Before we start with the discussion of wiring schemes for modular jacks, it is good to know how pins are numbered on RJ45 and other modular jacks. The following scheme shows the exact pin numbering on both male and female RJ45 connectors.

All other modular jacks—like RJ11—start counting at the same side of the connector. In the wiring diagrams with modular jacks on this site we prefer to use a picture of the jack upside down, with the hook underneath.

The straight through RJ45 network cable, EIA/TIA 568B

The most common wiring for RJ45 cables is the straight through cable. In this cable layout, all pins are wired one-to-one to the other side. The pins on the RJ45 connector are assigned in pairs, and every pair carries one differential signal. Each line pair has to be twisted. If UTP or FTP cable is used, the pairs have orange, brown, blue and green colors. The wiring of these cables to RJ45 connectors to make a straight through cable is defined by EIA/TIA 568B. The RJ45 connectors on both ends are wired in the same way. The color scheme is shown below.

The cross over RJ45 network cable, EIA/TIA 568A

The straight through RJ45 cable is commonly used to connect network cards with hubs on 10Base-T and 100Base-Tx networks. On network cards, pair 1-2 is the transmitter, and pair 3-6 is the receiver. The other two pairs are not used. On hubs, switches and routers, pair 1-2 is the receiver and 3-6 the transmitter. Because of this a straight through RJ45 cable can be used to connect network cards to hubs.

In very small network configurations where only two computers have to be connected, the use of a hub is not necessary. The straight through RJ45 cable cannot be used in that situation. Also when two hubs have to be connected to increase the number of nodes on a network segment, this cable is not appropriate. In both situations a cross over RJ45 cable is necessary, where the transmit and receive lines on both RJ45 connectors are cross connected. The color coding for the cross over RJ45 cable has been defined in the EIA/TIA 568A standard.

Please note: One RJ45 connector has to be wired as EIA/TIA 568B, the other as EIA/TIA 568A. When wiring both ends as EIA/TIA 568A, the resulting cable is a straight through cable again.

Common data and voice wiring schemes

Depending of the situation where modular cables are used, the wiring schemes with modular jacks differ. The most common wiring schemes can be seen in the picture below.