User Manual

Appendix C - Electrical Interface
Sealevel Systems COMM+8.LPCI Page 8
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 20K bps / 50 ft. The absolute maximum data rate may vary due to line conditions and
cable lengths. RS-232 often operates at 38.4K bps 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) denote 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 Sealevel Systems Adapter is a DTE interface.