Omron V550-A20, Operation Manual

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5
V550-A20 Operation Manual
Section 5-
5
Appendices
04 Wake up This message type is sent by the master. The
slave that receives it should acknowledge the
message.
05 No data This message type may be sent by a slave
indicating that the slave has no data to send in
response to a poll. This message is optional. If
the slave has no data, it may ignore the poll.
SEQ (1 ASCII digit) = The sequence number
This field starts at zero at power up, and is incremented by one for
each data message sent. When the sequence number reaches nine, it
wraps around to one. This field is only present in a data message.
DATA = The content of the data field
This field contains data, if the message type indicates that data is
included. This field may contain no characters (length of zero; poll,
acknowledge and wake up messages do not have data fields.)
LRC (2 HEX digits) = The Linear Redundancy Check Sequence
The LRC is computed by exclusive-oring all the characters in the ID,
TYPE, SEQ, and data fields, then converting the hex number into two
ascii digits. This mathematical process checks to make sure that the
message is valid.
CR (0Dh) = Carriage return
This character indicates the end of the message. When this character is
received, the unit should check to see that the message started with a STX,
and check that the LRC is correct before accepting it as a valid message.
Message Sequencing
The master unit initiates all data transfers by either sending data to a slave or
requesting data from a slave. This protocol is strictly half duplex; only one
device may be transmitting at any time. A slave device should not transmit
unless it receives a valid message that requires a response--when it does
receive such a message, it must respond quickly (See Timing). The master
unit should respond in a timely manner, but is not under the same con-
straints as a slave. The following is the example of processing a Master/
Slave interaction:
Master Slave’s response Master’s response
1. Wake up ACK -none-
2. Poll Data ACK
3. Poll No data -none-
4. Poll -none- -none-
5. Data ACK -none-