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2-PORT RS-232/422/485 PCI HOST ADAPTER
Appendix C: Electrical Interfaces
C.1 RS-232
Quite possibly the most widely used communication standard is RS-232. This
implementation has been defined and revised several times and is often referred
to as EIA/TIA-232.
The IBM PC/AT computer defined the RS-232 port on a 9-pin D-subminiature
connector, and subsequently the EIA/TIA approved this implementation as the
EIA/TIA-574 standard. This standard is defined as the 9-Position Non-Synchronous
Interface between Data Terminal Equipment and Data Circuit-Terminating
Equipment Employing Serial Binary Data Interchange. Both implementations are
in widespread use and are referred to as RS-232 in this document.
RS-232 is capable of operating at data rates up to 20 kbps at distances less than
50 ft. The absolute maximum data rate may vary due to line conditions and cable
lengths. RS-232 is a single-ended or unbalanced interface, meaning that a single
electrical signal is compared to a common signal (ground) to determine binary
logic states. The RS-232 and the EIA/TIA-574 specification define two types of
interface circuits: Data Terminal Equipment (DTE) and Data Circuit-Terminating
Equipment (DCE). The Adapter is a DTE device.
C.2 RS-422
The RS-422 specification defines the electrical characteristics of balanced-voltage
digital interface circuits. RS-422 is a differential interface that defines voltage levels
and driver/receiver electrical specifications.
On a differential interface, logic levels are defined by the difference in voltage
between a pair of outputs or inputs. In contrast, a single-ended interface, for
example RS-232, defines the logic levels as the difference in voltage between a
single signal and a common ground connection.
Differential interfaces are typically more immune to noise or voltage spikes that
may occur on the communication lines. Differential interfaces also have greater
drive capabilities that allow for longer cable lengths. RS-422 is rated up to
10 megabits per second and can have cabling 4000 feet (1219.2 m) long. RS-422
also defines driver and receiver electrical characteristics that will allow 1 driver and
up to 32 receivers on the line at once. RS-422 signal levels range from 0 to +5 volts.
RS-422 does not define a physical connector.