TIA-232
recommends using a separate wire (not one of the wires in the connector) to
connect the grounds of the two chassis if needed.
:
RS-232 logic levels are defined as positive and negative voltages rather than the
positive-only voltages of TTL and CMOS logic (Table 4-2). At an RS-232 data
output (TX), logic 0 is defined as equal to or more positive than +5V, and logic
1 is defined as equal to or more negative than -5V. In other words, the data uses
negative logic, where the more positive voltage is logic 0 and the more negative
voltage is logic 1.
Inside RS-232
47
The status and control signals use the same voltages, but with positive logic. A
positive voltage indicates a logic 1 and a function that is On, asserted, or True,
and a negative voltage indicates a logic 0 and a function that is Off, not
asserted, or False.
An RS-232 interface chip typically inverts the signals and converts between
TTL/CMOS voltages and RS-232 voltages. On a UART??™s output pin, a logic-1
data bit or an Off control signal is a logic high, which results in a negative voltage
at the RS-232 output. A logic-0 data bit or asserted control signal is a logic
low at the UART and results in a positive voltage at the RS-232 output.
Because an RS-232 receiver can be at the end of a long cable, by the time the
signal reaches the receiver, the voltage may have attenuated or have noise riding
on it.
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