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© Sealevel Systems, Inc.
3612 Manual | SL9115 9/2021
Appendix C
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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 (usually +3 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.
