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ASCII C
OMMUNICATIONS
A-3
Solutions with Vision
ASCII C
OMMUNICATIONS
S
TANDARD
RS485
M
ULTIDROP
C
OMMUNICATIONS
RS485 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 transmitters
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 requesting
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 RS485 communications
works:
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).
RS485 Communications Example
The remainder of this section defines the message formats and timing
requirements for the protocol used on RS485 multidrop (2-wire) communications
lines. The protocol is defined for both the “master” device and the “slave”
devices. This protocol is defined for a one-master system only. The following
definitions may help you understand this protocol a little better.
ASCII digit
This means the ASCII code for a single decimal digit. For example, 30h is the
ASCII digit that encodes a zero.
HEX digit
This means the ASCII code for a single hexadecimal digit. Some examples are,
35h is the code for a five, 42h is the code for a "B" (which equals 11 base 10), the
hexadecimal number "5A" would be encoded by the two HEX digits 35h and 41h.
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