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13.2.4 Data Communication from Printer to Host
13.2.4.1 Nibble Mode
In this mode, data is transferred between the printer and the host through the
procedure described below. The steps beginning from 1) are applicable when the
Negotiation phase has switched to the Host Busy Data Available phase. If the
Negotiation phase has switched to the Host Busy Data Not Available phase, the
procedure starts at step 9).
1) After the negotiations for the entry into Nibble mode are completed, the host
sets HostBusy (nAutoFd) to Low to indicate that it is ready to receive data from
the printer. (Event 7)
2) The printer places the low-order four bits on the reverse channel data line and
sets PtrClk (nAck) to Low. (Events 8 and 9)
3) The host sets HostBusy (nAutoFd) to High to indicate that it has latched data
and received the signal in event 9. (Event 10)
4) The printer sets PtrClk (nAck) to High. This completes transfer of the first
nibble. (Event 11)
5) Steps 1) to 3) are repeated to transfer the high-order four bits, before
proceeding to steps 6) and on.
6) After the host has set HostBusy (nAutoFd) to High (Event 10) and received data,
the printer must set the four status lines as shown below. (Event 13)
• PtrBusy (Busy): Returned to the status given in FORWARD MODE.
• nDataAvail (nFault): Set to Low if there is data to be sent.
• AckDataReq (PError): Set to Low if there is data to be sent.
• Xflag (Select): Set to the current mode (i.e., set to Low).
7) The printer sets PtrClk (nAck) to High. (Event 11)
8) After event 11, the host checks the signals set by the printer in event 13. With
this check the host determines:
1. Whether there is more data to be sent from the printer to the host;
2. And whether data can be transferred from the host to the printer.
9) If there is no more data to be sent from the printer after the transfer of one byte
(Two nibbles), the host chooses one of three status selections:
1. Performing Termination and returning to the Compatibility mode.
2. Remaining in the Host Busy Data Not Available phase.
3. Setting HostBusy (nAutoFd) to Low (Event 7) and moving to the Reverse Idle
phase.
10) If there is more data to be received from the printer, the host chooses one of
three status selections:
1. Setting HostBusy (nAutoFd) to Low and indicating that the host is ready to
receive.
2. Remaining in the Host Busy Data Available phase.
3. Performing Termination and returning to the Compatibility mode.
11) If the host selected the Host Busy Data Available phase and set HostBusy
(nAutoFd) to Low, the printer repeats the steps from 2) onwards.