Dell OptiPlex 755 - Manual (Page 129)

DC Power Connector P2
DC Power Connectors P3, P5, P8, and P9
DC Power Connector P7
DC Power Connector P10
19 GND Black
20 NC NC
21 +5 VDC Red
22 +5 VDC Red
23 +5 VDC Red
24 GND Black
*Optional wire. Use 22-AWG wire instead of 18-AWG wire.
Pin Number Signal Name 18-AWG Wire
1 GND Black
2 GND Black
3 +12 VADC Yellow
4 +12 VADC Yellow
Pin Number Signal name 18-AWG Wire
1 +3.3 VDC Orange
2 GND Black
3 +5 VDC Red
4 GND Black
5 +12 VBDC White
Pin Number Signal Name 22-AWG Wire
1 +5 VDC Red
2 GND Black
3 GND Black
4 +12 VDC Yellow