Lenovo ThinkSystem SR550 Product Manual page 33

I/O expansion
The SR550 server supports one LOM card slot and up to six PCIe slots: one PCIe slot on the system
planar and up to five PCIe slots with different riser cards installed into two riser sockets on the system
planar (one riser socket supports installation of one riser card).
The slot form factors are as follows:
LOM card slot
Slot 1: PCIe 3.0 x16 or PCIe 3.0 x8; full-height, half-length (PCIe x16 slot is double-wide)
Slot 2: PCIe 3.0 x8; full-height, half-length (not present if the slot 1 is PCIe x16)
Slot 3: PCIe 3.0 x8 or ML2 x8; full-height, half-length
Slot 4: PCIe 3.0 x8; low profile (vertical slot on system planar)
Slot 5: PCIe 3.0 x16; full-height, half-length
Slot 6: PCIe 3.0 x8; full-height, half-length
Notes:
Slot 5 requires the second processor to be installed.
Slot 4 is not present if the COM Port Upgrade Kit is installed.
The locations of the PCIe slots are shown in the following figure.
Figure 8. Slot locations
Lenovo ThinkSystem SR550 Server 33