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© Sealevel Systems, Inc. 5104e & 5104es Manual | SL9273 8/2021
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 (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.
RS-485
RS 485 is backwardly compatible with RS 422; however, it is optimized for party line or multi drop
applications. The output of the RS 422/485 driver is capable of being Active (enabled) or Tri State (disabled).
This capability allows multiple ports to be connected in a multi drop bus and selectively polled. RS 485
allows cable lengths up to 4000 feet and data rates up to 10 Megabits per second. The signal levels for RS
485 are the same as those defined by RS 422. RS 485 has electrical characteristics that allow for 32 drivers
and 32 receivers to be connected to one line. This interface is ideal for multi drop or network environments.
RS 485 tri state driver (not dual state) will allow the electrical presence of the driver to be removed from
the line. Only one driver may be active at a time and the other driver(s) must be tri stated. RS 485 can be
cabled in two ways, two wire and four wire mode. Two wire mode does not allow for full duplex
communication and requires that data be transferred in only one direction at a time. For half duplex
operation, the two transmit pins should be connected to the two receive pins (Tx+ to Rx+ and Tx- to Rx-).
