Applies To:
  • CitectSCADA x.x

I wish to Multi-drop GE PLCs using the SNP protocol, but it doesn't seem to work.

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.