Epson TM-U950P - B/W Dot-matrix Printer Specification page 154

J.2.4.3 Notes
1) The start of negotiation phase shall be defined with 1284-Active triggered.
2) The timing to start negotiation in the Compatibility Mode shall be in principle after nAck
pulse has been developed following nStrobe.
Upon detection of 1284-Active before or during the output of nAck after nStrobe , the
interface enters the negotiation phase. In this case, when the termination is followed by
the Compatibility Mode, nAck pulse is not developed.
3) Negotiation is available from Busy or Error state in the Compatibility Mode. In this case,
the termination fails to lead to the original Busy or Error state but to the printer status after
termination.
4) If the printer fails to support the communication mode requested by the host, the host shall
enter the Termination phase then return to the Compatibility Mode. (Refer to G.2.4.2.)
* The printer is designed to work with only Nibble/Byte and thus it responds with negative Xflag
values for request for the other modes.