Asus RS500A-S6/PS4 - Manual (Page 60)

4-12 Chapter 4: Motherboard information
10. BMC header (BMC_FW1)
The BMC connector on the motherboard supports an ASUS ® Server
Management Board 4 Series (ASMB4).
9. Serial port connector (10-1 pin COM1, COM2)
These connectors are for the serial (COM) ports. Connect the serial port
module cables to these connectors, then install the modules to the slots
opening at the back of the system chassis.
The serial port module is purchased separately.