Appendix C – Electrical Interface
Sealevel Systems
COMM+4.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 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
Sealevel Systems 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.
