How to tell old optiplex 745/755/760/780 BTX motherboard form factor

Edit: 745/755, almost all form factor (MT, DT, SFF, excluding USFF) use standard 24 pin ATX connector (white)

USFF is a different animal with no PCIex16 slot.  745/755/760 USFF use proprietary 8 pin external AC adapter. 780 USFF uses standard AC power cord but has proprietary 8 pin power connector on motherboard.

A quick way to tell optiplex 790 MT motherboard from 790 DT board, which can look alike on casual glance.

MT has 4 sata ports: blue (sata 3.0), black, white x2

DT has 3 sata ports: blue (sata 3.0), black, white x1 

790MT

790DT

MT/DT both have only one sata 3.0, which is sata_0 in BIOS.

MT/DT both have 2 PCIex16 (blue wired x16, black wired x4), 1 PCIex1, 1 PCI.

790SFF has three sata ports like DT but just two PCIex16 slots

790 SFF

@redxps630   Not sure why you are showing a metal box with 8 pin connector.

Your picture is NOT the power supply for a Dell Optiplex SX280 GX620 745 755 760 USFF  220W DA-2 power supply. Number of pins on a connector in no way implies a standard or compatability.  Radomly plugging in cables that go in is NOT RECOMMENDED.  No Dell model for several years has used ATX 20 pin power supply.  EPS12v 24 pin is NOT ATX 12v 20 pin.

The USFF uses a huge 220W black plastic power brick that looks like a laptop brick.

Sense jumpered to remote turns this PSU on.

DA2 Pinout to be used as aux video power

Amazon ASIN: B07Q6C1NX6

https://www.amazon.com/dp/B07Q6C1NX6

DELL DA-2

The way to tell is by the PPID aka Dell Part number for the board.

Sales on amazon and ebay can sometimes have incorrect pictures.

optiplex 745/755/760/780 BTX motherboard PPID numbers

https://www.dell.com/support/kbdoc/en-us/000139589/
What is the PPID number? PPID (Piece Part Identification) numbers
are affixed to specific Dell parts as a means to identify and or track individual components.

Dell PPID numbers can be utilized to determine the age and the part number of a particular part if that part may require warranty service by technical support.

Every Dell PPID number contains five separate sets of information for a total of 20-digits.

Figure 1

• Country of Origin (underlined blue in Figure 1)
• Dell Five-Digit Part Number (the first character is a filler number since all Dell part numbers are five digits, underlined green in Figure 1)
• Manufacture Identification
• Date Code (Year, Month, Date) (underlined red in Figure 1)
• Alphanumeric Manufacture Sequence Identification Revison Number

• Field	Description
  Part #	Part number of the component
  Part Desc	Description of the part
  Desc	Description of the commodity
  PPID	Piece Part Identification Number of the component
  Country Code	Country of origin
  Vendor Code	Identifier for the manufacturer
  Date Code	Year, month, and date of manufacture
  Serial Num	Alphanumeric manufacture sequence identification number
  Rev Level	Revision level of the part

It seems quite likely that Dell removed one white sata port (sata_3 in BIOS) from 790 MT board, which makes it DT.  790MT and DT have identical rear port layout.

In other words, MT sans sata_3 = DT motherboard.  One can still see the ghost foot print of sata_3 on DT board.  Just another OEM board design quirk.  It makes sense because 790MT accommodates up to two sata hdd and two sata odd, DT fewer, so why leave extra sata port on DT board.

(790SFF have different rear port layout from MT/DT.)

This discussion serves as a quick visual guide to motherboards of various old optiplex 700 series form factors,