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13.2 Negotiation
13.2.1 Overview
Usually, the printer is started in Compatibility mode, which corresponds to the
Centronics interface phase. When the host intends to switch to the Nibble or Byte
mode, it sends a request and negotiates with the printer. A general flow of
negotiations is given below.
1) The host sets the IEEE 1284 Active signal to High. In response, the printer moves
into the Negotiation phase.
2) The printer replies whether it can execute the mode requested by the host.
3) The interface terminates the Negotiation phase and moves into the
communication phase.
13.2.2 Negotiation Procedure
The negotiations proceed as follows:
1) In IEEE 1284 communication phase, the host and printer are in Compatibility
mode by default. They remain in Compatibility mode as long as the host
recognizes the connected device as an IEEE 1284 compatible device.
2) To start negotiations, the host sets the communication mode request bit on the
data path. (Event 0)
3) The host sets IEEE 1284 Active (nSelectln) to High, and HostBusy (nAutoFd) to
Low. (Event 1)
4) The printer responds by setting PtrClk (nAck) to Low, nDataAvail (nFault) to
High, Xflag (Select) to High, and AckDatReq (PError) to High. (Event 2)
5) The host sets HostClk (nStrobe) to Low. In response, the printer latches the data
of the communication mode request bit. (Event 3)
6) The host sets HostClk (nStrobe) and HostBusy (nAutoFd) to High. (Event 4)
7) If the printer has communication data to send to the host, it sets AckDataReq
(PError) to Low, nDataAvail (nFault) to Low, and Xflag (Select) to the value
corresponding to the communication mode. (Event 5)
Xflag: Nibble Mode: Low,
Byte Mode: High
8) The printer sets PtrClk (nAck) to High to indicate that it is ready to read status
lines. (Event 6)
9) If the printer has communication data to send to the host, the host moves into
the Host Busy Available phase or Termination phase, and then returns to the
Compatibility mode.
10) If the printer has no communication data to send to the host, the host moves
into the Host Busy Data Not Available phase or Termination phase, and then
returns to the Compatibility mode.
11) If the printer cannot support the communication mode requested by the host, it
sets Xflag (Select) as follows:
When Nibble mode is requested:High
When Byte mode is requested: Low