PCB-mount modular jacks place a modular plug interface directly on an electronic assembly. They are used in telephones, network equipment, controls, test equipment, modems, embedded systems and OEM products that need a field-accessible modular connection.
Although terms such as RJ11 and RJ45 are commonly used, board-jack selection should be based on the actual position/contact count, mechanical footprint, orientation and electrical design. An 8-position jack may be used for Ethernet or for a completely different serial/control pinout.
Choose the jack by positions and contacts, top- or side-entry orientation, PCB footprint, mounting/retention features, shielding and whether integrated magnetics or indicator LEDs are required.
Verify the contact configuration, such as 6P4C, 6P6C or 8P8C, and compare the PCB hole pattern or surface-mount footprint to the board design.
Orientation matters: right-angle side-entry and vertical top-entry jacks place the cable differently relative to the PCB and enclosure opening.
For Ethernet designs, determine whether the jack is a simple modular connector or a MagJack-style component with integrated magnetics. These are not interchangeable design functions even when both accept an 8P8C plug.
Where EMI control is needed, shielded jacks can provide a conductive shell connection to chassis or board ground when the equipment design supports it. Also verify latch direction and panel clearance so the plug can be inserted and released after assembly.