Impulse ACB-104 (3512) - Manual (Page 18)

Appendix C - Electrical Interface
Sealevel Systems ACB-104 Page 15
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. The driver is in a tri-state or high impedance
condition when this occurs. Only one driver may be active at a time and the other driver(s) must be tri-stated. The
output modem control signal Request to Send (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.