Omega OMG-COMM8-PCI - Manual (Page 15)

Appendix C - Electrical Interface
OMG-COMM8-PCI Page 11
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 appro ved 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 OMG-COMM8-PCI
adapter is a DTE interface.