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Appendix C - Electrical Interface
Appendix C - Electrical 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 partyline 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. The output modem control
signal RTS controls the state of the driver. Some communication software
packages refer to RS-485 as RTS enable or RTS block mode transfer.
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.
Sealevel Systems Dual SIO
Page 12
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