Chapter 6. Serial I/O, SNP & RTU Protocols
256 PACSystems* RX7i, RX3i and RSTi-EP CPU Reference Manual GFK-2222AD
6.3.6 RTU Slave/SNP Slave Operation with Programmer Attached
A port that has been configured for RTU Slave protocol can switch to SNP protocol if an SNP master such
as a programmer begins communicating to the port. The programmer must use the same serial
communications parameters (baud rate, parity, stop bits, etc.) as the currently active RTU Slave protocol
for it to be recognized. When the CPU recognizes the SNP master, the CPU removes the RTU Slave
protocol from the port and installs SNP Slave as the active protocol.
The SNP protocol that is installed in this case has the following fixed characteristics:
â–ª The SNP ID is set to blank. Therefore, the SNP master must use a blank ID in the SNP attach message.
This also means that this capability is only useful for point-to-point connections.
â–ª The turnaround time is set to 0ms.
â–ª The idle timeout is set to 10 seconds.
After the programmer is removed, there is a slight delay (equal to the idle timeout) before the CPU
recognizes its absence. During this time, no messages are processed on the port. The CPU detects
removal of the programmer as an SNP Slave protocol timeout. Therefore, it is important to be careful
when disabling timeouts used by the SNP Slave protocol.
When the CPU recognizes the programmer disconnect, it reinstalls RTU Slave protocol unless a new
protocol has been configured in the meantime. In that case, the CPU installs the new protocol instead.
Example
1. COM1 is running RTU Slave protocol at 9600 baud.
2. A programmer is attached to COM1. The programmer is using 9600 baud.
3. The CPU installs SNP Slave on COM1 and the programmer communicates normally.
4. The programmer stores a new configuration to COM1. The new configuration sets the port for SNP
Slave at 4800 baud (it will not take effect until the port loses communications with the programmer).
5. When the CPU loses communications with the programmer, the new configuration takes effect.
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