©Sealevel Systems, Inc.
SL9222 07/2011
Relio R5200 Manual
76
Appendix F – Serial Interfaces
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.
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 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-). Four wire mode allows full duplex data transfers. RS-485
does not define a connector pin-out or a set of modem control signals. RS-485 does not define a
physical connector.
