microATX Motherboard Interface Specification

Version 1.2

Page 23

3.4.3 3.3V Sense Line

A remote 3.3 V sense line can be added to the optional connector to allow for accurate control of the 3.3VDC

line directly at motherboard loads. Because of potential voltage drops across the connector and traces leading

to the motherboard components, it can be advantageous to implement a 3.3V sense line that remotely monitors

the 3.3VDC power level at the load on the motherboard. The implementation of this signal should be such

that if an NC condition is detected on this line, the default 3.3V sense line on the main connector would be

used for sensing the 3.3 VDC voltage level.

3.4.4 1394V Pin

This pin on the optional connector allows for implementation of a segregated voltage supply rail for use with

unpowered IEEE-1394 solutions. The power derived from this pin should be used to power only 1394

connectors. The output of this power rail depends on the 1394 compatibility required. Use of this rail for

motherboard or other power needs can have unpredictable results, because power for 1394 devices is not

required to be regulated and can provide voltage levels between 8 and 40 volts. See the applicable IEEE-

1394 specification for details on the specific power requirements for this rail. If this rail is implemented, it

should operate such that the main PS-ON signal must be asserted low for power to be delivered at this

connector.

3.4.5 1394R Pin

The 1394R pin provides an isolated ground path for unpowered 1394 implementations. This ground should

be used only for 1394 connections and should be fully isolated from other ground planes in the system.