AMT Datasouth XL 300 SERIES, Maintenance Manual

Page: 19 / 117

2.3: CHARACTER PRINTING
Incoming characters are taken from the communications interface (either serial or parallel)
and placed into a FIFO buffer. The characters are then placed into a print buffer to
construct the next line to be printed. No printing occurs until all data for that line of print
has been received and the printer receives a line terminator (e.g. line-feed, vertical tab,
form feed). Upon receipt of the line terminator, the microprocessor examines the present
speed and position of the printhead. This information along with the format of the new
print line enables the microprocessor to select the optimum print direction (hence the term
“logic seeking”). The microprocessor examines the first character in the print buffer to be
printed, retrieves the dot pattern for this character from ROM and places the dot pattern in
a dot buffer. The dot pattern is then loaded into a printwire latch. When the printhead
achieves the correct position, the dot pattern is sent to the printhead power amplifier. Seven
sub-columns are required to completely define each character. Dots in each sub-column are
formed by energizing solenoids which cause the wires in the printhead to impact the ink
ribbon and paper.
2.4: PRINTHEAD THEORY OF OPERATION
The printhead contains nine individual solenoids which must be energized selectively.
Each solenoid has a darlington transistor with which to select it. The length of time which
the solenoids are energized is critical to printhead performance. This length of time is
determined by a mono-stable multi-vibrator which enables the darlington transistors for a
fixed amount of time.
When the solenoids are energized they are driven in a voltage mode. There are three
distinct, sequential periods which occur. (See Figure 2.2).
Figure 2.2 Printhead Solenoid Current Waveform
2-2