This is a standard for the motherboard of a PC/AT compatible machine.
The standard that follows in the wake of the specifications of the motherboard used in the IBM PC/AT machine defines the AT specifications.
The power supply is started by directly switching ON the AC supply, so it is highly resistant to malfunctioning due to noise, for example, and is used even today in industrial applications.
The size of the power supply is the same as that of the ATX, namely W 150 x H 86 x D 140 (mm) (PS/2 size).
The output connectors differ from those of the ATX in that the main connectors consist of two 6-pin connectors.