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Appendix C - Electrical Interface
Omega Engineering OMG-COMM8-A Page 15
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-C/D/E or EIA/TIA-232-C/D/E. It is defined as
“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. The
IBM PC computer defined the RS-232 port on a 9 pin D sub connector and
subsequently the EIA/TIA approved this implementation as the
EIA/TIA-574 standard. This standard has 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 wide spread use and will be
referred to as RS-232 in this document. RS-232 is capable of operating at
data rates up to 20 Kbps / 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
(usually +3 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 define two types of interface circuits Data Terminal
Equipment (DTE) and Data Circuit-Terminating Equipment (DCE). The
Omega Engineering Adapter is a DTE interface.