Omega Vehicle Security ISA RS-422 - Manual (Page 19)

Appendix C – Electrical Interface
OMG-ULTRA-SIO Page 15
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. 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.