The Parallel Port: Negotiation Sequence

A 1284 compliant peripheral device must implement the negotiation sequence, because this is how reverse channel and advanced modes are activated.

		/Select		/AutoFeed		D0,...,D7		/Strobe		/Ack		/Error		Select		PE
		1284Active		HostBusy				HostClk		PtrClk		/DataAvail		XFlag		AckDataReq
1																		No extended 1284 mode must be active
2																		H ensures sane state of control lines
3																		H places negotiation byte on data lines
4																		H requests negotiation
5																		P acknowledges negotiation
6																		H strobes negotiation byte into P
7																		H waits (for >= 0.5us?)
8																		H requests answer
9																		P places abilities on bus
10																		P signals valid answer
11

Notes

• /Init must be constantly high during negotiation.

• If the /Strobe signal is missing, a peripheral should return to the de-selected Centronics compatibility state when /AutoFeed goes high again. There is a proper 1284 termination sequence, too.

The negotiation byte is a combination of bits that the host uses to negotiate a mode, select a sub-device, and possibly request a device ID from the peripheral.