User Manual

Appendix C – Electrical Interface
Sealevel Systems ULTRA COMM+422.PCI Page 17
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.