COMHP 7404/-D25
CyberResearch
®
Serial Communications
12
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CyberResearch, Inc.
Appendix B - 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 (u3 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
D
ata
T
erminal
E
quipment
(
DTE
) and
D
ata
C
ircuit-Terminating
E
quipment (
DCE).
The 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.
