Citizen Systems iDP3240 - Manual (Page 185)

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(1) Termination through a handshake between the host and the printer:
When switching from Reverse mode to Compatibility mode, this termination
method can be used if the interface is activated (IEEE 1284 Active: High) and
event 22 has taken place.
1) The printer responds to IEEE 1284 Active by setting PtrBusy (Busy) and
nDataAvail (nFault) to High. (Event 23)
2) The printer then inverts Xflag (Select) and sets PtrClk (nAck) to Low.
(Event 24)
3) The host sets HostBusy (nAutoFd) to Low. (Event 25)
4) The printer returns nDataAvail (nFault), Xflag (Select), and AckDataReq
(PError) to the status given in the Compatibility mode, and sets PtrClk (nAck)
to High. (Events 26 and 27)
5) The host sets HostBusy (nAutoFd) to High to terminate the handshake and
return the interface to the Compatibility Mode Idle phase. (Event 28)
6) The printer changes PtrBusy (Busy) to be able to receive data from the host.
(2) Immediate termination:
1) If the interface is deactivated (IEEE 1284 Active: Low) without event 22
having taken place, the printer immediately performs Termination. In this
termination, the data is not guaranteed, and the printer switches the data
bus from output to input within 1 µsec.
In the Reverse Idle phase, the printer can notify the host that it has data to transfer
to the host. The notification may occur simultaneously with termination in order
for the host to move from the Idle phase to the Compatibility mode.
If the printer has data to send, it initiates the Interrupt phase indicated by events 8
and 9. In this case, if 1284 - Active (nSelectIn) was set to Low before HostBusy
(nAutoFd) changed from High to Low, the printer interprets that the host has
switched to the Termination phase, and then completes the normal termination
through handshaking.