[Transfering data between PC (Slave) and PLC (Master) using MODBUS tcp/ip over Ethernet.]

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 192.168.192.5 on port 502. My laptop is set to 192.168.192.10  (subnetmask 255.255.255.0, gateway 192.168.192.254)

 

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: 192.168.192.10 

Destionation: 192.168.192.5 

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: 192.168.192.5  

Destionation: 192.168.192.10

Protocol: TCP 

Length: 60 

Info: asa-appl-proto > 49933 [ACK] Seq=1 Ack=325 Win=1024 Len=0

 

Followed by

Source: 192.168.192.10 

Destionation: 192.168.192.5

Protocol: TCP 

Length: 64 

Info:  [TCP segment of a reassembled PDU]

 

And finally

Source: 192.168.192.5  

Destionation: 192.168.192.10

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