With the ASUS Z87-C motherboard the setup prompts and messages of bootable add-in cards (including the Realtek onboard ethernet) are not visible on the VGA monitor output, but only on the DVI output
This was tested with version 1602 of the MB BIOS and several add-in cards, including an Intel Pro NIC (PCI bus), and an Adaptec RAID adaptor (PCIE). In addition, when the CMOS is placed in PXE Boot mode the messages from the Realtek NIC are not visible.
The CMOS setup code displays correctly on either monitor port.