Impulse SIO-104+16.485 (3562) - Manual (Page 17)

© Sealevel Systems, Inc. - 15 - SIO-104+16.485 User Manual
Appendix C – Electrical Interface
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-). 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.