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5.2.3 Negotiation
Overview
Usually,theprinterisstarted in Compatibility mode, whichcorresponds tothe Centronics interface phase.
WhenthehostintendstoswitchtotheNibbleorBytemode,itsendsarequestandnegotiateswiththeprinter.Ageneralflowof
negotiations is given below.
(1) The host sets the IEEE 1284Active signal toHigh. In response,the printer moves intothe Negotiation phase.
(2) The printerreplies whether itcanexecute themode requested by thehost.
(3) The interface terminates theNegotiation phase andmoves intothe communication phase.
NegotiationProcedure
Thenegotiations proceedasfollows:
(1)InIEEE1284communicationmode,the hostandprinterareinCompatibilitymodebydefault.They remain inCompatibility
mode aslong as the hostrecognizes theconnecteddevice asanIEEE 1284compatible device.
(2) To start negotiations,the host sets thecommunication moderequest bit on thedata path. (Event 0)
(3) The host sets IEEE 1284Active (nSelectln) toHigh,andHostBusy(nAutoFd)toLow.(Event 1)
(4) The printerrespondsby setting PtrClk(nAck) toLow, nDataAvail (nFault)toHigh, Xflag (Select)toHigh, andAckDatReq
(PError) toHigh. (Event 2)
(5) The host sets HostClk (nStrobe) to Low.In response,the printer latchesthe data of thecommunicationmode requestbit.
(Event3)
(6) The host sets HostClk (nStrobe) and HostBusy(nAutoFd)toHigh. (Event 4)
(7) If theprinterhas communication data tosendtothehost,itsets AckDataReq (PError) toLow,nDataAvail (nFault) toLow,
and Xflag (Select)to thevalue corresponding tothecommunicationmode. (Event 5)
Xflag: Nibble Mode :Low
Byte Mode :High
(8) The printersets PtrClk (nAck) toHighto indicate that it isready toread status lines. (Event 6)
(9)Iftheprinterhascommunicationdatatosendtothehost,thehostmovesintotheHostBusyAvailablephaseorTermination
phase,andthenreturns tothe Compatibilitymode.
(10)Iftheprinter hasnocommunication datatosendtothehost,thehost movesintothe HostBusyDataNot Available phase
orTermination phase, andthen returns totheCompatibility mode.
(11) If theprinter cannot support thecommunicationmode requested bythe host, it sets Xflag (Select)asfollows:
When Nibble mode is requested : High
When Bytemode isrequested : Low
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