What is a PCI slot? What are its uses?

what Are PCI Slots? pci slot meaning PCI-X is generally backward-compatible with most cards based on the PCI or later standard, meaning that, a PCI-X card can be installed in a PCI slot pcie 30 x4 slot General Specifications Name: AMD Ryzen™ 5 5600X · Connectivity PCI Express® Version: PCIe® · Graphics Capabilities Graphics Model: Discrete Graphics Card

pcie slot size ASUS mining motherboard features 19 PCI Express® slots With 18 PCIe x1 and one PCIe x16 slots on board, plus three zoned ATX12V power connectors