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Appendix C: Communication Devices
approximately 20 m maximum, it is still widely used. This is due to its
simplicity and low cost.
Electrical
TIA/EIA-232 has high signal amplitudes of ±(5 V to 15 V) at the driver
output. The triggering of the receiver depends on the sign of the input
voltage: that is, it senses whether the input is above 3 V or less than –3
V. The line length is limited by the allowable capacitive load of less
than 2500 pF. This results in a line length of approximately 20 m. The
maximum slope of the signal is limited to 30 V/ms. The intention here
is to limit any reflections that can occur to the rise-and fall-times of the
signal. Therefore, transmission line theory does not need to be applied,
so no impedance matching and termination measures are necessary.
Do not connect termination resistor when operating in RS-232 mode.
Protocol
Different from other purely electrical-layer-standards, TIA/EIA-232
defines not only the physical layer of the interface (ITU-T V.11), but
also the pinout of the appropriate connectors (25-pin D-type or 9-pin
DB9S) (ISO 2210) and the protocol (ITU-T V.24). The interface
standard specifies also handshake and control lines in addition to the 2
unidirectional receive data line (RD) and transmit data line (TD). The
control lines might be used, but do not necessarily have to be.
RS-232 is Single-Ended Point-to-point Transmission
Single-Ended, Point-to-Point
Single-ended transmission is performed on one signal line, and the
logical state is interpreted with respect to ground. For simple, low-
speed interfaces, a common ground return path is sufficient; for more
advanced interfaces featuring higher speeds and heavier loads, a single
return path for each signaling line (twisted pair cable) is recommended.
The figure below shows the electrical schematic diagram of a single-
ended transmission system.
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