Chapter 3. CPU Configuration126 PACSystems* RX7i, RX3i and RSTi-EP CPU Reference Manual GFK-2222AD3.2.10 COM1 and COM2 ParametersThese parameters configure the operating characteristics of the CPU serial ports68. COM1 and COM2have the same set of configuration parameters. The protocol (Port Mode) determines the parametersthat can be set for each port.Port ParametersPort Mode The protocol to execute on the serial port. Determines the list of parameters displayed on thePort tab. Only the parameters required by the selected protocol are displayed.Choices:▪ RTU Slave mode: Reserved for the use of the Modbus RTU Slave protocol. This mode alsopermits connection to the port by an SNP master, such as the WinLoader utility or theprogramming software.▪ Message mode: The port is open for user logic access. This mode enables C language blocksto perform serial port I/O operations via the C Runtime Library functions.▪ Available: The port is not to be used by the PLC firmware. (The CPE302/CPE305 does notsupport this selection.)▪ SNP Slave: Reserved for the exclusive use of the SNP slave. This mode permits connection tothe port by an SNP master, such as the WinLoader utility or the programming software.▪ Serial I/O: Enables you to perform general-purpose serial communications by usingCOMMREQ functions.Default: RTU Slave.Note: If both serial ports are configured for any protocol other than RTU Slave or SNP Slave,the RUN/STOP Switch should not be disabled without first making sure that there is away to stop the CPU, or take control of the CPU through another device such as theEthernet module. The Serial I/O protocol is only active when the CPU is in RUN Mode. Ifthe CPU can be set to STOP Mode, it will switch the protocol from Serial I/O to the STOPMode protocol (default is RTU Slave). If an SNP Master, such as the programmingsoftware in Serial mode, begins communicating on a port, the RTU protocolautomatically switches to SNP Slave. As long as the CPU can be stopped, the protocol ofthe port can be auto-switched to one that enables serial programmer connection. Referto STOP Mode protocols.If an Ethernet port is available, you can communicate with the CPU by connecting PMEsoftware via the Ethernet port.StationAddress(RTU Slave only) ID for the RTU Slave.Valid range: 1 through 247.Default: 1.Note: You should avoid using station address 1 for any other Modbus slave in a PACSystemscontrol system because the default station address for the CPU is 1. The CPU uses thedefault address in two situations:1. If you power up without a configuration, the default station address of 1 is used.2. When the Port Mode parameter is set to Message Mode, and Modbus becomes theprotocol in STOP Mode, the station address defaults to 1.In either of these situations, if you have a slave configured with a station address of 1,confusion may result when the CPU responds to requests intended for that slave.Note: The least significant bit of the first byte must be 0. For example, in a station address of090019010001, 9 is the first byte.68 Not supported by RSTi-EP CPE100/CPE115 at the time of publication.