Information Technology Reference
In-Depth Information
Data flow
Host
system
Station
1
Station
2
Station
32
Figure 14.1
RS-485 connecting to multiple nodes
RS-485 operates in one of two modes:
•
Two-wire, multidrop, party line - in this mode, a balanced transmission line is used to
connect to all of the stations, which share a common communications channel. Up to 32
driver/receiver pairs can share the common channel.
•
Four-wire - in this mode, each station connects to a four-wire bus, as illustrated in Fig-
ure 14.2. It is necessary in this mode that one node acts a master station and all others as
slaves. The master then communicates with each of the slaves. All slave nodes commu-
nicate only with the master node. A master-slave network is useful when mixed proto-
cols are used.
Terminating
impedance
Master
Station 1
Station 32
Figure 14.2
RS-485 connecting to multiple nodes
The RS-485 four-wire connection involves a half-duplex transmission mode, that is, only one
device can transmit data at a time. This thus involves a polling procedure, with a master and
up to 32 slaves. The slaves must wait in a high-impedance state (a stand-by mode). The con-
trol of the driver on the slave is either with:
