RJ45 Connectors – What You Need to Know

After their introduction in the 1960s by AT&T to replace bulky phone connectors, modular connectors have been a staple of communication and data services worldwide. While initially targeting phone connections, registered jack (RJ) connectors expanded over the years to include data communication as well. This led to what might be the most recognized modular connector today, the RJ45 connector, which is used in computer networking applications and is commonly referred to as an Ethernet connector. Although widely used, this article aims to provide an in-depth overview on all things RJ45 connectors, including their basic features and capabilities, applications, standards, and more.

RJ45 basics

At a most basic level, RJ45 connectors are modular interconnection devices paired with a cable that deliver data communication to various Electronic systems. They contain 8 contacts and 8 wire positions utilized for signals or power, meaning they allow for 4 twisted wire pairs. Although this 8-contact, 8-position setup makes them similar to other 8P8C connectors, actual RJ45 connectors further feature a tab that only allows insertion of the device in one orientation with a compatible socket to prevent improper connections. While most 8P8C connectors are referred to as RJ45, this is not entirely accurate because 8P8C connectors can plug into RJ45 sockets, but RJ45 connectors cannot plug into 8P8C sockets.

As with any modular connector, RJ45 connectors offer a variety of benefits, such as low cost, solderless assembly of connector and wiring, quick production of custom cables, simple insertion and removal, easy field assembly with simple tooling, and the ability to customize cables on-site. Outside of these basic benefits, RJ45 connectors can also offer several additional features including:

  • Shielding – cancels the effects of EMI/RFI
  • Keying – ensures proper insertion
  • Various mounting capabilities – allows for panel, board, surface or through hole methods
  • Displays and indicators – indicates connection status
  • Integrated magnetics – allows for better shielding plus current protection
  • Hi-Rel – provides protection for the hardware and the connection

RJ45 uses and applications

RJ45 connectors are most commonly used to connect one internet-enabled device with another network device. For example, a PC connected to a server, router, modem, smart TV, gaming console, or any device utilizing Ethernet protocol. The hardwired connection that RJ45 connectors provide allows for greater data speeds, improved stability, and higher security. This makes RJ45 connections attractive for both professional use and personal use.

RJ45 Connectors – What You Need to KnowFigure 1: RJ45 connectors are found in a range of networking devices (Image source: CUI Devices)

Added features and capabilities, such as increased speed and ruggedness, to the RJ45 product line has also made this interconnection system applicable for uses outside of office or home networking, while the introduction of the EtherCAT protocol has led to increased use on the factory floor and beyond. Some of the newer applications that use RJ45 technology include:

  • Factory Automation
  • Industrial Process Control
  • Industrial Robotics (fabrication and assembly)
  • Test and Measurement Systems
  • Quality Control Systems
  • Voice Over Internet Protocol (VOIP) Systems and Devices
  • Internet of Things (IoT) Network Devices

Intro to RJ45 connector standards

There are several applicable standards for RJ45 connectors, depending on their application and intended use. These applicable standards are briefly summarized here, but several standards will be outlined in greater detail later:

  • ANSI/TIA 1096-A – covers the basic mechanical characteristics, physical dimensions, and contact requirements for RJ45 devices
  • T-568A and T-568B – the basic wiring and pinout standards
  • IEEE 802.3at, 802.3af and 802.3bt – the various power over Ethernet (PoE) standards for RJ45 connectors capable of supplying electrical current to the end device
  • IEEE 1394 – data interface bus structure used with RJ45

A variety of other standards define the cables or wires utilized for RJ45 devices, again largely depending on the end application.

Ethernet and EtherCAT

RJ45 and Ethernet are two terms that go hand in hand. Developed by IEEE in the 1980s, the Ethernet standard (IEEE 802.3) defines the rules for configuring an Ethernet network (connections), the number of conductors required for a connection, the expected performance, and a data transmission framework. It remains the most widely used and popular LAN technology around the world due to its ease of use, cost, speed, and broad network protocol support with RJ45 connectors being the standard devices utilized in network set-up.

Introduced in 2003, Ethernet for Control Automation Technology (EtherCAT) is a highly flexible network protocol used by the industry for real-time factory automation, test and measurement, and more. It is defined by IEC standard 61158 and operates at high speeds with great efficiency. EtherCAT networks utilize two RJ45 ports with no other special hardware required. One RJ45 is connected to the previous node in the network and the other is connected to the next node, which allows for efficient use of bandwidth.

Ethernet cabling categories

Cabling used in Ethernet and EtherCAT networks is offered in several different types from Cat5 and Cat6 to now Cat7 and even the new Cat8. Each of the cable standards offer greatly different performance metrics, but all still utilize the same RJ45 connectors. This means that each cable, regardless of the standard, will consist of 8 wires and 4 twisted wire pairs, which matches the RJ45’s 8P8C configuration.

Cat5 is mostly obsolete at this point thanks to Cat5e cables that are up to 10 times faster than Cat5 and offer better resistance to crosstalk or interference. Next, Cat6 improves the bandwidth capacity over Cat5e, further reduces crosstalk and interference via shielding, and remains backwards compatible with Cat5/5e. It is often used in network installations in need of Gigabit speeds. The newer Cat6a incorporates a thick plastic casing for an even further reduction of crosstalk and allows for longer cable runs up to 328 feet. Cat7 and Cat7a each incrementally increase bandwidth capacity.

Cable Type Maximum Speed Maximum Bandwidth
Cat 5 100 Mbps 100 MHz
Cat 5e 1,000 Mbps 100 MHz
Cat 6 1,000 Mbps 250 MHz
Cat 6a 10,000 Mbps 500 MHz
Cat 7 10,000 Mbps 600 MHz
Cat 7a 10,000 Mbps 1,000 MHz

Table 1: Ethernet cable category specifications (Image source: CUI Devices)

Power over Ethernet (PoE)

In addition to relaying data signals, RJ45 connectors offer the ability to deliver electrical power to the connected devices by taking advantage of the unused twisted wire pairs in Ethernet cables. This Power over Ethernet (PoE) technique is defined by IEEE standard 802.3af and its iterations that continue to raise the power delivery capability.

Nomenclature Standard Maximum Power
PoE IEEE 802.3af Up to 15.4 W
PoE+ IEEE 802.3at (Type 2) Up to 30 W
PoE++ IEEE 802.3bt (Type 3) Up to 60 W
PoE++ IEEE 802.3bt (Type 4) Up to 100 W

Table 2: Power over Ethernet standards (Image source: CUI Devices)

The ability to supply DC power over these wire pairs eliminates the need for separate power wiring. With no need for separate electrical cables, outlets, or installation, PoE has grown rapidly thanks to those provided cost savings and flexibility. PoE is also designed to be safe, reliable, and scalable.

RJ45 Connectors – What You Need to KnowFigure 2: PoE injectors provide power-to-Ethernet connected devices (Image source: CUI Devices)

T568A and T568B standards

T568A and T568B are the two RJ45 pinout standards that specify the assignment of the 8 wires within the connector. These color-coded wires must be correctly inserted into their specified pinout locations for proper functioning of an Ethernet network. T568A offers backward compatibility for older wiring and T568B delivers better signal isolation and noise protection. RJ45 pass-through connectors allow the wires to be fed through the connector and trimmed during the crimping process for easier alignment.

RJ45 Connectors – What You Need to KnowFigure 3: T568A and T568B pinouts (Image source: CUI Devices)

RJ45 Connectors – What You Need to KnowFigure 4: T568B wiring shown in an RJ45 connector (Image source: CUI Devices)

Deciding on which standard is appropriate will depend on individual design needs and whether a straight-through cable or crossover cable is being used. Straight-through (patch cable) has the same wiring standard at each end, while crossover cable connects the same types of devices by having T568A at one end and T568B at the other end. Rolled or rollover cables are flat and connect devices to a network switch console port. They are only used to establish an interface and do not transmit data.

Loopback and T1 are two other common terms that may be heard. Loopback is a type of connection that allows a computer to interface with itself, which might be utilized for diagnostics, troubleshooting, and connection with servers. T1 lines, while much less common than 15-20 years ago, are dedicated lines directly from a telecom service provider to an end-user, which offer higher speeds and can carry both voice and data.

Final design considerations for RJ45

RJ45 connectors that will be interfacing with an Ethernet network have several technical considerations to keep in mind during design. At the board level, the focus is on reducing EMI, preserving electrical signal isolation, and maintaining signal integrity. Without diving too far into the details of circuit design, it is important to pay close attention to the length and position of traces on the board. Physical placement of the connectors on the PCB should also be considered.

When it comes to magnetics, they must be considered if adding wired Ethernet into a design as they are included in the Ethernet specification for 10/100/1000 Base-T networks. Magnetics are wire-wound components (like transformers) that provide EMI shielding, protect against faults and transients, and offer electrical isolation and signal balancing. They must be isolated and kept as close as possible to RJ45 connectors.

There are two options for adding magnetics into a design. Option one is to select an RJ45 connector with integrated magnetics in the housing. RJ45 magnetic jacks offer the benefits of improved EMI shielding and more reliable connections. The second option is to add a magnetics module to the PCB between the PHY (Ethernet implementation) chip and the regular RJ45 jack. Magnetics modules tend to be cheaper than integrated connectors while offering better electrostatic discharge (ESD) protection. Both options have advantages and disadvantages that will need to be weighed based on the overall system design.


Like other modular connectors, RJ45 connectors offer easy design-in, quick and easy installation, breadth of product, and user accessibility. They have been accepted worldwide for both home and office network applications and are finding increasing acceptance on the factory floor and in harsh environments.

CUI Devices offers a wide range of RJ45 connectors featuring a variety of options including integrated magnetics, PoE, LED indicators, and more.