Figure 23 Printer Communications Buffer Flow - TransAct Ithaca 90PLUS Series Programmer's Manual

Programmer's Guide
In most cases, the host computer is capable of sending information to the printer much faster than the printer can
print it. To prevent information from being lost, a flow control mechanism is provided. This mechanism is called
the flow control protocol. The goal of the flow control protocol is to exchange as much information as possible
as fast as possible without losing any data. The Series 90PLUS Printer supports three flow control protocols,
two in serial mode and one in parallel.
From the printer's point of view, there are four basic functions that are required of communications. All four are
common to all three flow control protocols. There must be a communication driver, status inquire procedure,
storage buffer, and print control mechanism that is using the data.
1. The communication port is either the serial port hardware or the parallel port hardware and the associated
communication software driver.
2. A means for the host to bypass the buffer for status information is also required because the buffer offsets
the printer in time from the host. (The printer is generally behind the host).
3. The storage print buffer is a software implemented, first-in first-out circular buffer. It stores information in
an asynchronous fashion where information can be placed in it at any rate and retrieved from it at any rate.
The information order, however, is not altered. Any buffer has a finite size, and if information is put in
faster than it is taken out, it will overflow. To avoid overflow, a flow control mechanism is required.
4. The print control mechanism is the remainder of the printer hardware and control software. It interprets
control codes and operates the control panel, print head, and cash drawer interface.
The following figure illustrates the four basic parts of printer flow control.
Com m unic atio ns
port
S erial or p arallel
Data
c onnec tion
S tatus

Figure 23 Printer communications buffer flow

The communication port is either serial or parallel and is controlled by a software communication driver. The
driver receives data and returns requested status. As information is received from the host, data is taken from the
communication port hardware by the driver, preprocessed to look for status inquire commands, and placed in the
buffer. When the buffer input function finds that the buffer is getting full, it notifies the communication driver to
implement flow control. Flow control will attempt to stop further information from being sent from the host.
11/07/05
PcOS Series 90PLUS
P rin te r
E NQ
proc es s o r P rint buffer
Data
Data
S tatus
G etting full/
getting em pty
Rev Q
Communications
P rint c ontrol
Data
P rint head
Overview
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