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About dpeterrr

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  1. I'm trying to connect my PC with a Unitronics PLC. I would like to use Modbus over TCP/IP for this purpose. This is the first step in my project. Later on I would like to connect it to a Rabbit Wolf (BL2600) to exchange I/O. I first want to connect the Unitornics PLC to my laptop to get a decent understanding of the Modbus TCP/IP protocol used by Unitronics. At the moment I'm already stuck, so I hope that someone could help me. I did read the documentation about Modbus Application Protocol on: http://www.modbus.org/docs/Modbus_Application_Protocol_V1_1b.pdf I'm using the Unitronics example code: V280_Ethernet_TCP_MODBUS_IP_Master (without any changes) I used the Vellamod tool to connect and send data: http://www.tuomio.fi/vellamod/index.htm (I'm running windows 8, so I have some problems with other tools and this one seems to work nice) I'm using Wireshark to inspect the TCP/IP traffic. However, I'm now in the current situation. The laptop connects, but I don't get any response. I connect to on port 502. My laptop is set to (subnetmask, gateway I try to send the following (see attachment for a printscreen): byte 0: transaction identifier = 0 byte 1: transaction identifier = 0 byte 2: protocol identifier = 0 byte 3: protocol identifier = 0 byte 4: length field (upper byte) = 0 byte 5: length field (lower byte) = 2 byte 6: unit identifier = 255 byte 7: MODBUS function code = tried 3 (Read Holding Registers) and 4 (Read Input Register) byte 8: 00 byte 9: 26 (I try to read MI 38) When I than inspect the transmission with WireShark, I get the following Source: Destionation: Protocol: Modbus/TCP Length: 64 Info: Query: Trans: 0; Unit: 255, Func: 3: Read Holding Registers[Malformed Packet][Malformed Packet] When I set: byte 5: length field (lower byte) = 6 I get no error, but I also receive nothing with the Vellamod tool. According to Wireshark I receive the following: First Source: Destionation: Protocol: TCP Length: 60 Info: asa-appl-proto > 49933 [ACK] Seq=1 Ack=325 Win=1024 Len=0 Followed by Source: Destionation: Protocol: TCP Length: 64 Info: [TCP segment of a reassembled PDU] And finally Source: Destionation: Protocol: TCP Length: 60 Info: asa-appl-proto > 49933 [ACK] Seq=1 Ack=335 Win=1024 Len=0 On the PLC HMI nothing is changing (Socket 3 total transmits: 0;Socket 3 total reiveces: 0) The only strange thing that I can see is that it (Unitronics) seems to use destination port 49933, but I don't know whether this is wrong or right. Hopefully someone can help me. Visual basic code, would also be more than welcome. Kind regards, Peter
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