Appendix D – Electrical Interface
Omega Engineering
OMG-P232/P422
Page
18
Appendix D - 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. 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 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 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 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
Omega Engineering 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.