Figure 32 Host To Printer Link - TransAct POSjet 1500 Programmer's Manual

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Programmer' s Guide POSjet 1500 Data Processing and
Communications
Data to printer
Printer
Host computer
Status and flow control back

Figure 32 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
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fast as possible without losing any data. The POSjet 1500 Printer supports three flow control protocols, two in
serial mode and one in parallel.
The printer requires four basic functions to achieve successful 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.
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