Rockwell Automation 1799-DASCII DeviceNet Serial Interface - Manual (Page 26)

26 DeviceNet Serial Interface Module
Publication 1799-IN012B-EN-P - July 2008
In this mode the two required for the complete transaction are the following:
•
A "New Data Available" flag is set by the 1799-DASCII module in the status byte. This informs the
master that the 1799-DASCII module has received a new data string and is waiting for the OK to send
it.
•
The first byte of the message the master sends to the 1799-DASCII module is the RX ID Record. By
setting the value of this byte to the value of the new record number pending, it indicates to the
1799-DASCII module that the master is ready to receive the new data.
The master monitors this new data flag and when the master is ready to receive
new serial data, it sets a new number in the new record number byte of the next
poll command message. This applies only to data being sent from the 1799-DASCII
module to the master.
The operation proceeds as follows.
1. The 1799-DASCII module receives a new data string.
2. The 1799-DASCII module sets the New Data flag in the Status byte of its next
produce message.
3. The master sends out messages to the 1799-DASCII module in the normal
fashion. If the master is not ready to receive new data, the New Record Data
byte remains set.
4. When the master is ready to receive the new data string, it changes the New
Record Data byte to any value different than what it had been sending.
5. The 1799-DASCII module sends the new data upon receipt of a record from
the master in which the New Record Data byte has been changed.
6. If the 1799-DASCII module receives an updated New Record Data byte and
has no new data, it sets the Handshake Error bit in its Produce Status byte.
In Immediate mode, this handshaking is not active and the 1799-DASCII module
sends new data as soon as it is received from the ASCII device with the new
Transaction ID. It is the master's responsibility to be ready to accept and process
the new data string when it is presented.
Identify the Scanner I/O Receive Size (parameter 15)
The 1799-DASCII module automatically calculates the number of bytes it sends the
DeviceNet master. Its value is determined by a combination of the incoming data
and the options you selected. Parameter 15 is a read-only value that defines the size
of the DeviceNet message to be sent by the 1799-DASCII module to the master.