Control Codes
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Printer Status Status Inquire
The Epic 880TM Printer is designed for use as part of an automated system where the
host computer makes every attempt to correct problems with the printer. In addition,
the host application requires that it be able to obtain more information from the printer
than is typical of normal computer printers. For example, since a normal computer
printer does not have cash drawers, such additional features require that the standard
printer protocol be extended to deal with the added features of a point-of-sale (POS)
printer.
All inquire commands are processed as they are received (preprocessed or real time)
and require a response from the printer. Consequently, parallel, IEEE 1284
bidirectional communications, USB or bidirectional serial operation is required.
The Epic 880TM Printer looks at and evaluates all commands as they are received,
and does not respond to inquire commands that happen to be embedded in graphics
or other commands. For more details, refer to the buffer and preprocessor
descriptions in later sections.
In all cases, inquire commands are responded to by an acknowledged (ACK) or a not
acknowledged (NAK) and then the command ID, which allows the host application to
make multiple requests and receive identifiable responses. If the printer is configured
for serial or USB operation the status is automatically returned to the host. If the
printer is configured for parallel, IEEE 1284 operation, the HOST must initiate a
reverse channel request to return the status.
Serial and USB Mode Inquire
All inquire (ENQ) commands require a response from the printer. During serial
operation, all inquire commands are responded to by an acknowledged (ACK) or not
acknowledged (NAK), the command ID, and in some cases status. Most status
responses sequences contain a length field to help decode and separate responses.
In general the printer should be configured for “Buffer Full Only” off-line operation if
inquire commands are used. This prevents the printer from using flow control for
anything but buffer full. The programmer must take on the responsibility for assuring
that inquire commands are used to maintain status of the printer.
The printer always accepts serial data even when it is off-line. The printer has reserve
buffer space that allows additional information to be received even if the printer is
signaling buffer full or off-line. Because inquire commands are processed before they
go into the buffer, the printer responds even when it is busy printing.
In serial mode, the response to an inquiry should be received by the host before
another inquire command is issued to the printer. When the printer receives an
inquiry, it generates a response. If inquiries are sent to the printer too quickly, the
printer spends all of its time responding and does not have time to print.