Appendix C – Electrical Interface
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
RS-232 or EIA/TIA-232. It is defined by the EIA as the Interface between Data
Terminal Equipment and Data Circuit- Terminating Equipment Employing Serial
Binary Data Interchange. The mechanical implementation of RS-232 is on a 25 pin
D sub connector. 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 often operates at 38.4 Kbps over very short
distances. The voltage levels defined by RS-232 range from -12 to +12 volts.
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. A
voltage of +12 volts (u3 to +10 volts) represents a binary 0 (space) and -12
volts (-3 to -10 volts) denotes a binary 1 (mark). The RS-232 and the EIA/TIA-574
specification defines two type of interface circuits, Data Terminal Equipment (DTE)
and Data Circuit-Terminating Equipment (DCE). The Sealevel Systems adapter is a
DTE interface.
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 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.
© Sealevel Systems, Inc.
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ACB-MP+4.PCI User Manual
