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2
V550-A20 Operation Manual
Section
5-
2
Appendices
MASTER
Slave
Slave
Slave
Slave
Slave
Standard RS-485 Multidrop Communications
RS-485 communications is an Engineering Industries Association standard
for the transmitters and receivers of a digital equipment interface. RS485
communication uses differential signal lines and allows for multiple trans-
mitters on one signal pair (although only one transmitter may be enabled at
any given time). This is a way of allowing one device to communicate with
one or more other devices using the Master /Slave method.
The Master/Slave system works as follows:
The master device (usually a decoder logic or computer) originates poll
messages. The poll message is a message from the master to a slave request-
ing the slave to respond with data (if data is available). The slave is usually
a bar code scanner. The slave device responds to the polls from the master.
It is not allowed to transmit unless it has been “asked” (polled) by the
master.
Shown below is a simplified drawing of one way that RS-485 communica-
tions works:
NOTE:
This representation shows one Master and six slaves. You can
ultimately have up to 32 slaves for each serial port on the master (depending
on the line length and required response time).
Slave
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