Chapter 6. Serial I/O, SNP & RTU ProtocolsGFK-2222AD April 2018 2576.4 SNP Slave ProtocolPACSystems CPUs can communicate with Machine Edition software through either COM1 or COM2 usingSNP slave protocol.CPU COM1 is wired as an RS-232 Data Communications Equipment (DCE) port, and can be connecteddirectly using straight-through cable to one of the serial ports of a PC running Machine Edition or otherSNP master software.CPU COM2 is wired for RS-485. If the SNP master does not have an RS-485 port, an RS-485/RS-232converter is required. The RX3i can use converter IC690ACC901, which uses +5Vdc from the serial port.The RX7i CPU COM2 does not support IC690ACC901 and requires an externally powered converter.PACSystems provides the break free version of SNP, so that the SNP master does not need to issue abreak signal as part of the SNP attach sequence. However, the CPU responds appropriately if a breaksignal is detected, by resetting the protocol to wait for another attach sequence from the master.PACSystems supports both point-to-point connections (single master/single slave) and multi-dropconnections (single master/multiple slaves).For details on SNP protocol, refer to the Series 90 PLC Serial Communications User’s Manual, GFK-0582.CPE400 does not support this protocol.6.4.1 Permanent DatagramsPermanent datagrams survive after the SNP session that created them has been terminated. This allowsan SNP master device to periodically retrieve datagram data from many different controllers on a multi-drop link, without the master having to establish and write the datagram each time it reconnects to thecontroller.The maximum number of permanent datagrams that can be established is 32. When this limit is reached,additional requests to establish datagrams are denied. One or more of the permanent datagrams willneed to be cancelled before others can be established. Since the permanent datagrams are notautomatically deleted when the SNP session is terminated, this limit prevents an inordinate amount ofthese datagrams from being established.Permanent datagrams do not survive a power-cycle.6.4.2 Communication Requests (COMMREQs) for SNPThe PACSystems serial ports COM1 and COM2 currently do not provide SNP Master service, nor do theysupport COMMREQ functions for SNP commands. However, those COMMREQ functions can be used withPCM/CMM modules that are configured to provide SNP service. For more information, refer to the Series90 PLC Serial Communications User’s Manual, GFK-0582.
