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Bench Talk for Design Engineers

Bench Talk


Bench Talk for Design Engineers | The Official Blog of Mouser Electronics

Demystifying the RJ Connector Rudy Ramos

The registered connector, better known as the RJ connector and commonly known as a modular connector, is one of the most frequently used connectors, present in almost every business and household in America. Yet, it is also one of the most misunderstood, and frequently plagues its users with doubts as to whether they are using the correct connector for their application.

Before I try to shed some light into the RJ series connector and the vast number of variations that encompass the series, let me first say that the RJ connector has been the de facto connector for the telecommunication industry for a number of decades. It has been the “work horse” when it comes to electronic applications that require the transmitting and receiving of data. For instance, in the Information Technology (IT) world, these connectors can be abundantly found attached to every server, in every server room throughout corporate America. They’re also attached to every LAN terminal, copier, fax, and wireless access point. In homes, they connect phone landlines, and bring the internet to the masses via, modems, routers and computers. When you think twisted pair cabling you intuitively also assume the use of RJ connectors for the termination. The combination of these non-mutually exclusive parts, connector and twisted pair wire, deliver the overall capacity of the interconnect that will meet any specification.

Many new connector types have recently made their way into the marketplace, more specifically into the consumer market. For instance, USB and HDMI, both very good connectors, but in my opinion both have little chance of displacing the RJ connector anytime soon. The main reasons for this are multiple. First, the small form factor makes them ideal for IT applications. Second, cost and availability of these connectors. You can buy a handful of these connectors and find them at just about any hardware supplier for under a $1 each. And lastly, the ability to terminate these connectors with ease. Not to mention the acceptance by OEMs as the standard connector for IT applications. Fast and easy termination of these connectors makes them ideal for new installations and field service repairs.


Figure 1: Typical server connections

The following information is by no means exhaustive, but it does provide a short list to get you started on selecting the right connector for the job.

So why is there so much confusion figuring out what connector to use for your application or what connector you have? Perhaps is because over time the RJ connector has evolved to meet the demands for greater and faster through puts of data. Some of these changes have only required more pins to be added to existing versions of the connector while other changes have increased the number of physical pins the connector houses. In both cases, the connectors themselves look very similar to each other while their actual specifications could be drastically different.

I’ll try to demystify the RJ connector starting with some basic information and examples of common industry abbreviations and acronyms. Note that some of these don’t only apply to the connector but also to the wire that is used with these connectors. The more you know about the connector-wire combination the better off you’ll be at understand what connector you’ll need for your application.

Most of the time, you will find these acronyms and suffixes attached to the connector part number or description.


Table 1: RJ connector nomenclatures

Many of the basic names have suffixes that indicate subtypes:

• C: flush-mount or surface mount

• F: flex-mount

• W: wall-mount

• L: lamp-mount

• S: single-line

• M: multi-line

• X: complex jack

List 1:

Acronyms for twisted wire:

 Table 2:

Figure 2:

Table 3:

Table 4:

I hope you have found this handy information on RJ connectors useful and as previously stated, there is much more to these connectors that I’ve omitted, including various standards and certifications, recommended wire runs lengths, recommended bend radius, etc. For standard landline phone service, RJ 11 connectors would be fine. For most residential audio video and data applications, Cat.5 UTP wire with RJ45 connectors is probably the standard. More demanding commercial applications, CAT.5e or better would probably fit the ticket with special consideration to cable shielding, cable stranding, plenum approved or not and possibly the cable jacket insulation material.

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Rudy is the Project Manager for the Technical Content Marketing team at Mouser Electronics, accountable for the timely delivery of the Application and Technology sites from concept to completion. He has 30 years of experience working with electromechanical systems, manufacturing processes, military hardware, and managing domestic and international technical projects. He holds an MBA from Keller Graduate School of Management with a concentration in Project Management. Prior to Mouser, he worked for National Semiconductor and Texas Instruments. Rudy may be reached at

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