Seagate 18FC manual SEL2

26		Barracuda 18FC Installation Guide, Rev. A
	Pin/contact	Signal name	Signal type
	26	Ground
27*		+Port B_in	Diff. PECL input pair
	28*	–Port B_in
29		Ground
	30*	+Port A_out	Diff. PECL output pair
31*		–Port A_out
	32	Ground
33*		+Port B_out	Diff. PECL output pair
	34*	–Port B_out
35		Ground
	36	SEL_2	TTL input/output
37		SEL_1	TTL input/output
	38	SEL_0	TTL input/output
39		Reserved NC
	40	5 Volts charge

*Short pins in mating backpanel connector.

[1]This pin may be connected to external logic to detect the presence of the drive. The drive connects this pin to the common ground.