Internet PLC Forum

Hi,
We are in the process of designing a Modbus slave web service over Modbus tcp but we are getting strange behavior on the plc side. Hope you can help us find the problem.

Here's the PLC code

S = STATUS(3)
if(S = 1) THEN PRINT #4 "</>" ENDIF
 
  //CONNECT TO THE REMOTE DEVICE
  PRINT #4 "<MBTCPCONNECT IPAddress:502>"
  Call wait4conn  //WAIT FOR CONNECTION
  //GET CONNECTION STATUS
  S = STATUS(3)  //S=1 = CONNECTED, S=0 = NOT CONNECTED      
   

   IF S = 1 THEN
      setlcd 2,1, "Reading TEMP...      "  
      READMB2 4,1,40004,DM[151],2
      //DM[153] = ReadModbus(4,1,4)
      N$ = "Waiting response....."
      Call WaitForDataInD
      IF STATUS(2) = 0 THEN
         f$ = input$(4)
         DM[151]=999
         DM[152]=999
         setlcd 2,1, "Reading TEMP failed!!!!   "
      ELSE
         setlcd 2,1, "Reading TEMP success!!!!!"
      ENDIF
       PRINT #4 "</>"
   ENDIF

The actual stream of two requests between the PLC and the slave service is below:

At the ModbusSlave web Service:
Recv From PLC: 006700000006 01 04 9C 44 00 02 | Send From WebSVC: 006700000007 01 04 04 00 00 42 34

Recv From PLC: 008D00000006 01 04 9C 44 00 02 | Send From WebSVC: 008D00000007 01 04 04 00 00 42 34

I can't see where is failing...

Thank You, I added the bolded while loop after finishing with the connection and it seems to be working fine now.


Code:

PRINT #4 "<MBTCPCONNECT "+I$+":502>"
Call Wait4conn  //WAIT FOR CONNECTION
S = STATUS(3)  //S=1 = CONNECTED, S=0 = NOT CONNECTED
   
   IF(S = 1) THEN
      setlcd 2,1, "READING REMOTE DEV..."
      READMB2 4,Q,1000+(O-1),DM[N],VAL(M$)
       IF STATUS(2) = 0 THEN
          setlcd 2,1, "BAD READ ADDRESS..."
          save_eep$ "ERROR READING "+STR$(1000+(O-1)) +":" + STR$(VAL(M$))+" AT " + I$+"."+STR$(Q), 40+L
       ELSE
          setlcd 2,1, "GOOD READ ADDRESS..."
          save_eep$ "GOOD READING "+STR$(1000+(O-1))  +":" + STR$(VAL(M$))+" AT " + I$+"."+STR$(Q), 40+L
       ENDIF
       REFRESH  //REFRESH IO'S    
   ELSE
       setlcd 2,1, "UNABLE TO CONN REM..."
       save_eep$  "ERROR: "+INPUT$(4)+", UNABLE TO CONNECT "+I$+"."+STR$(Q), 40+L
   ENDIF


PRINT #4 "</>"  
while status(3) <> 0
//do nothing...
endwhile



Function Wait4Conn:
I = 1

while STATUS(3) <> 1 OR I > 1000
I = I + 1
ENDWHILE
S = STATUS(3)
I = 1
RETURN


Thanks,
Ed

You are 100% correct, we completely overlooked the buffer on the server side. Everything is working fine now...

Thanks,
Ed

Hello,

We are on the first stages of testing a new web service that will extend current PLC functionality via the web to interface with other web enable device. During the testing we are noticing that the Ethernet output buffer is not clearing after each write. Is there a function or a procedure to clear the buffer that we are missing?

Example:
//Send test string...
PRINT #4 "This is a long string that contains information about this PLC"
//Run your data read loop here....
//Send another string...
PRINT #4 "This is a great PLC"  //but when you look at the server side the text we receive is like this  "This is a great PLCg that contain information about this PLC"

We are getting the remaining char in the buffer "g that contains information about this PLC".

Before and after each command we tried the following loop but it still transferring the same.

While (len(X$) > 0)
   X$ = Input$(4)
endwhile

Anything else you can suggest? Currently, on the server side we are parsing the string up to the first CR.

Thanks,
Ed

Not sure I understand your question? The PLC comes with the data sheet that shows you the Modbus address table and reference of each PLC IO.

Ed

Thanks for the quick reasponse. The test im running, the data comes from a web mail server. The server terminates the data with CRLF and each segment are less than 70 chars long, however I think the data is arriving too fast or the nano internal buffer is over flowing and overwriting the strings. Is the internal buffer also has a max of 70 chars? Or is that even a possibility?

I can only retrieve the end of the message.

Thanks,
Ed

Thanks, I'll be using it to parse information requested from the internet as the time server example does. But my problem is that this file is about 1500 bytes too large.

Thanks,
Ed