| The SNP protocol specification seems to
allow Multi-dropping with the use of addressing. However, the SNP
protocol specification also requires the transmission of a long
break as the first step in establishing a communication session
with a given PLC. The long break generates a hardware interrupt on
all the CPUs on that serial link. In response to the long break,
all CPUs initialize and setup their communications hardware to wait
for an 'Attach' message from the master establishing the
connection. The 'Attach' message specifies which CPU the master
wishes to establish the communication session with. Only that CPU
will respond to the driver with the 'Attach-Response' message, and
all other CPUs on the serial link go back to a state that waits for
the long break interrupt.
Because of this design of the SNP protocol, it does not allow
for reliable support of multi-dropping with the high performance
expected of Citect users. Therefore Citect does not support
multi-dropping of the SNP protocol.
|
