TransAct Ithaca 8000, Programmer'S Manual

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Communications Ithaca
®
8000 Programmer’s Guide
Page 270 Rev D 100-06908
Host computer
Data to print er
Status and flow control back
Printer
Figure 14 Host to Printer Link
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. The 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 Ithaca
®
8000 printer supports three flow control
protocols, two in serial mode and one in parallel.
From the printer‘s point of view, four basic functions 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.
The communication port is either the serial port hardware or the parallel port hardware
and the associated communication software driver.
A means for the host to bypass the buffer for status information, referred to here as an
inquirer processor, is also required because the buffer offsets the printer in time from the
host. (The printer is generally behind the host).
The storage print buffer is a software implemented, first-in first-out (FIFO) 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, but the information order is not altered. All
buffers have a finite size, and if information is put in faster than it is removed, the buffer
will overflow. To avoid overflow, a flow control mechanism is required.
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.