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4
V550-A20 Operation Manual
Section
5-
4
Appendices
The following is framing for all messages sent by any device on the
multidrop line:
FFH STX ID(2) TYPE(2) SEQ DATA LRC(2) CR
(FFhex)
= Guard Character
This character is "sacrificed" to the line noise that occurs when the
unit transmitter is first turned on. The unit software may (optionally)
wait one character time between transmitter enable and transmission
of the STX (the next character). This eliminates transmitting the guard
character. The receiver ignores this character.
STX (02hex)
= Start of text character
This character indicates the start of a message. The receiver should
clear any characters in its receive buffer whenever it receives this
character.
ID (2 ASCII digits)
= The unit ID
This field indicates the unit identification number of the unit to which
the message is directed, if the message is coming from the master.
This field indicates the unit identification number of the unit transmit-
ting the message, if the message is from a slave.
A message with an ID of "00" from the master is a broadcast message.
All slave units should act on the message (display data, reset, etc.),
but no slave should respond to the message.
TYPE (2 ASCII digits)
= The message type
This field describes the purpose of the message that is sent. There are
five message types as shown below:
Message Types
01
Poll
This message type is sent by the master unit
to request data from a slave.
02
Data
This message type is sent by either a master to
transfer data to a slave or by a slave to transfer
data to the master after receiving a poll. The
TYPE field will then be followed by a SEQ field
and a data field.
03
ACK
This message type is sent by the unit that has
just received a valid data message.
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