Manual LPCI-COM-8SM
22
RS485 Data Transmission
The RS485 Standard allows a balanced transmission line to be shared in a party-line mode. As
many as 32 driver/receiver pairs can share a two-wire party line network. Many characteristics
of the drivers and receivers are the same as in the RS422 Standard. One difference is that the
common mode voltage limit is extended and is +12V to -7V. Since any driver can be
disconnected (or tristated) from the line, it must withstand this common mode voltage range
while in the tristate condition.
The following illustration shows a typical multidrop or party line network. Note that the
transmission line is terminated on both ends of the line but not at drop points in the middle of the
line.
Figure A-1:
Typical RS485 Two-Wire Multidrop Network
RS485 Four-Wire Multidrop Network
An RS485 network can also be connected in a four-wire mode. In a four-wire network it's
necessary that one node be a master node and all others be slaves. The network is connected
so that the master communicates to all slaves and all slaves communicate only with the master.
This has advantages in equipment that uses mixed protocol communications. Since the slave
nodes never listen to another slave's response to the master, a slave node cannot reply
incorrectly.
