Citizen Systems BD2-2220 - Manual (Page 305)

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Data Communication from Printer to Host
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.
(12) If the host selected the Reverse Idle phase and new data becomes available to be sent from the printer,
the printer sets PtrClk to Low to request the host for an interrupt. (Event 18)
(13) The printer sets PtrClk back to High. (Event 19)