PCOM over TCP

[selim]

Hi there;

I need to access MI area of a V1040 PLC through ethernet.

I had written a code that communicate with PLC via serial port before for an another project. Now, I am planning to modify my code to run under TCP.

At PC side, I am working with Delphi XE2 using Indy TCP client component. I tested these components on my network with another computers and they work properly.

At PLC side, I am using socket 1, port #20256 and Protocol is TCP Slave.. (I think these are default settings of socket 1 so I don't need to add a socket init FB to my ladder, am i right?)

PLC info mode says that card is initialized and socket is connected. In addition, when I send a PCOM string, RX counter increases which showed on info mode but the most important thing is missing: There are no replies.

The string I sent is a valid PCOM message which returns answer while I sent it over serial port and plus 6 bytes of ethernet header at the leading.

Can you please help me to figure out where is my mistake?

With my best regards

[Saragani]

I cannot see anything wrong with what you are doing,

But without knowing what exactly you send, I cannot know if you are doing anything wrong or not.

Can you post what you send (including the ethernet header)?

It can also be usefull to post a WireShark sniffing of the packets that are transfered.

[selim]

Yes Saragani, you were right! I was building correct packets but couldn't deliver them correctly to the PLC. And I figured out this just after "sniffing" the packets..

Let me explain why I can't send packets from Delphi to PLC:

Indy components are using blocking mechanism. Which means you write and read from a socket "synchronously". This simply just like a console application I/O.

I was using;

IOHandler.WriteLN()

method for sending my packet which accepts STRING as parameter. Therefore I was building my PCOM packet on a local Pascal string. And if you set a character value as zero it means your string is terminated. Although I was building correct packets, remote device had only seen first three characters because fourth character was always zero on the ethernet header..

To solve this problem, I did following:

    MsgBufferDump:  array[0..19] of byte =
   (
     $AA, $08,     // Transaction id = 2218
     101, 0,          // Protocol ID : 101 = ASCII
     14, 0,            // Payload size = 14
     47,                // STX: '/'
     48, 48,          // unit id = 00
     82, 87,          // Command: 'RW'  (Read MIs)
     0, 0, 0, 0,      // Vector base: will be filled by calling method
     53, 53,          // Vector length is fixed: 85 ($55)
     0, 0,              // checksum  : will be calculated just before sending
     13                 // ETX: <CR>
   );

Before sending, I copy this array to a stream:

sendstream:= TMemoryStream.Create;

sendstream.Write(MsgBufferDump,20);

sendstream.Position:= 0; // if you dont do this you will get an exception

IOHandler.Write(sendstream,20);

sendstream.Free;

And now it is running properly. Thank you very much for your interest.