Ithaca ITHERM 280, Programmer'S Manual

Page: 299 / 318

* % 8 %.
!
The serial port inquire is more straightforward than parallel mode. The serial
acknowledged (ACK) or not acknowledged (NAK) responses follow a uniform format, the
ACK or NAK is always followed by the command ID that requested it. This makes the
design of the host application easier because the response can be identified and always
follows the same format.
Flow Chart 3 Inquire Flow
The host sends an [ENQ] <id> (form sensor status) request to the printer. The printer
responds with an [ACK] <id> indicating that the request was identified, status true.
Inquire commands affect the printer’s performance. Communication is a high priority for
the printer. Inquire commands take processing time away from the print tasks. It is
possible to ask for so much status that the printer slows. In serial mode, it is important
that the response to an inquiry is received by the host before another command is
issued. When an inquiry is received by the printer, it is buffered in a high-speed
processing queue. When the printer has time, it empties the queue and processes the
inquiries. The printer responds to the command as soon as the second byte of the
command is taken from the high-speed buffer. If the host is looking for a form to be
inserted, it should not send status requests as fast as it can. The host will receive a
response to all of them. If the host did not wait for a response to each, there would be
unnecessary responses.
In IEEE 1284 mode, inquire responses are placed in an IEEE 1284 transmit queue.
When the IEEE 1284 reverse channel is open, the responses are returned to the host. It
is important that after each request the reverse channel be opened. Inquire responses
remain in the queue until read. If the [ENQ] <9> command is sent to the printer, the
IEEE 1284 buffer will be cleared, and only the response to the [ENQ] <9> will remain.
The host sends an
inquire to the printer
asking for the form
sensor status.
The printer receives
the request and
preprocesses it.
The printer responds
with an [ACK] <id> or
[NAK] <id> indicating
the form sensor
status.
The host receives the
response and
decodes it.