In fact, the WattNode will ignore two outstanding frames sent in rapid succession, since that is indistinguishable from a message collision or bus contention scenario. The specification reserves the first two bytes of the 6-byte TCP/IP header for a “transaction identifier”, or frame count, which is no longer needed when only one command message can be outstanding at a time.
However, it is possible to connect multiple WattNode meters via RS-485 to the same Ethernet Transaction Network Services (TNS) as long as the master software waits for the reply from a given WattNode meter before issuing a command to another WattNode. The WattNode meter does not support this since it can only process one command at time and it is usually the only end-point device.
This also alleviates the need for a frame count as discussed in the next bullet item. This eliminates any need for buffering multiple pending commands from the host (master) to the WattNode Modbus RS-485 slave device. Our implementation assumes that the master and slave devices are connected by a properly functioning LAN segment such as a corporate intranet where packet delivery and response latencies are kept well under a few seconds.
However, if the Modbus master includes a Modbus message checksum, the WattNode Modbus will verify it in received messages and include one with the replies. Since the TCP/IP packets that encapsulate the RTU messages have a CRC-32 (Cyclic Redundancy Check), the 8-bit Modbus message checksum was omitted in the standard.
Modbus TCP-RTU protocol is essentially the Modbus RTU serial protocol layered on top of (or encapsulated by) the TCP/IP protocol. If your application doesn’t require multiple concurrent client connections but it only supports the Modbus TCP/IP protocol then please read further to see how you can use a lower cost SDA or SDS instead of a Modbus gateway to reduce cost. The most significant difference is that Modbus TCP/IP supports multiple client connections at the same time whereas the latter approach only supports a single client connection at any given time. There are a few differences to be aware of between the Modbus TCP/IP protocol and the “Modbus RTU Serial via raw TCP/IP socket” approach.
Many Modbus master applications have another option (called Remote Telnet Server, Serial Device Server etc.) that allows sending unmodified Modbus RTU Serial messages through a TCP/IP socket to any Serial Device Server (SDS) or Serial Device Adapter (SDA), not just Modbus TCP/IP gateways. Note that Modbus TCP/IP is not the only way to do this. The Modbus TCP/IP protocol facilitates communication with Modbus TCP/IP gateways over an Ethernet LAN and the Internet. The WattNode ® Modbus ® meter’s TCP-RTU communications option emulates the Modbus TCP protocol over RS-485, allowing the use of low-cost serial-Ethernet gateways.