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P r i n t e r I n t e r f a c e I n f o r m a t i o n H P D e s k W r i t e r P r i n t e r s
Mac I/O Description
HP DeskWriter printers detect and monitor which interface mode (serial
or LocalTalk) is present, and automatically switch to the appropriate
protocol. The printers determine if they are connected to a LocalTalk
node by detecting the presence or absence of a transformer at the
interface connector port. (A transformer detected at the interface
connector indicates the printer is connected to an LocalTalk network.) If
the mode changes while the printer is on, the printer restarts.
HP DeskWriter printers are compatible with two Macintosh serial
interface modes:
RS-422-A (57.6 Kbaud)
Clocked
RS-422-A signals are modulated against an inverted copy of the
modulated signal (for example RxD- and RxD+). Data communication
occurs by sensing which line is more negative than the other. Sensing
line polarity in respect to an inverted signal is more immune to
interference and noise than to a signal compared to a common ground.
The signal is also less likely to degrade over longer cable distances.
If a serial connection is made and "clocked" mode is detected, the printer
switches from DTR mode (the default at power-up ) to SPP mode. The
printer selects a frequency for the clocking and clocks the DTR line at that
frequency.
In AppleTalk mode, the printer receives data in a synchronous serial bit
stream at 230.4 K bits per second and converts the data to parallel.
Note
The Macintosh I/0 connector is RS-422-A compatible. It is
not possible to use this port as an RS-232-C serial
interface port.