Dell Precision Workstation 390 Quick Reference Manual page 16

For single- and dual-monitor capable cards with a single connector
VGA Adapter
VGA
Use the VGA adapter when you have a single monitor
graphics card and you want to connect your computer
to a VGA monitor.
Dual VGA Y Cable Adapter
VGA
VGA
Use the appropriate Y cable when your graphics card
has a single connector and you want to connect your
computer to one or two VGA monitors.
The dual-monitor cable is color coded; the blue connector is for the primary monitor, and the black
connector is for the secondary monitor. To enable dual-monitor support, both monitors must be attached
to the computer when it starts.
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Quick Reference Guide
Dual DVI Y Cable Adapter
DVI
DVI
Use the appropriate Y cable when your graphics card has a
single connector and you want to connect your computer
to one or two DVI monitors.