Appendix C - Electrical Interface
Sealevel Systems
ACB-II
Page
13
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 or EIA/TIA-232. It is defined by the EIA as the
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.
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 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.
