2.5mm TRS vs. XLR 4-Pin

2.5mm TRS is the smallest common member of the phone-connector family -- the same tip-ring-sleeve wiring as 3.5mm TRS, scaled down to a 2.5mm shaft. It was used for headset jacks on cordless phones, two-way radios, and early portable tape recorder remotes before 3.5mm became the standard smaller size.

4-pin XLR uses the same connector shell as standard 3-pin XLR but carries entirely different, incompatible signals depending on the equipment: intercom headsets, balanced headphone outputs, or 12V DC power -- with no universal standard or keying to prevent connecting the wrong device to the wrong port.