plc comm.

Nikhil

Member
Join Date
Dec 2003
Posts
23
I had earlier posted a thread regarding pc to plc comm. I know that using the plc df1 protocol for an allen bradley micrologix 1500 allows one to manipulate or read data in the N9 integer user created file.
The comm. code based on df1 Protocol is written in VB. all the DLE And checksum errors have been gone thru more than thrice. Yet when i am reading the data of N9:0 and N9:1 into my PC from the Plc , I get an overflow after reading for a few seconds.
HAs anybody encountered this problem before. Please , i need to sort it out by tommorrow otherwise i am in for it.
 
This is just one of the reasons I always recommend buying an off-the-shelf protocol driver.

An overflow in what ? Is the controller returning an error code, or is your software failing to process the controller's data responses ?
 
Last edited:
Plc Comm. Reply

I get an overflow error message in windows. there is no error code from the Plc. I forgot to mention earlier that the code is able to Write easily into the N9 file i.e. if i want to set the N9:0/1 bit high, it does so easily, but when i want to read the same thing, it reads for a while then gives the overflow message. I have now checked this same code on another PC (just in case the comm. ports of my original PC were faulty) and also by changing the comm. cable and the channels 0 and 1 on the PLC. There too i get the same problem.
Is there any setting reqd. on the PLC side besides configuring the ports for DF1 full duplex protocol and the CRC bits alongwith the stop bits, NAK retries, etc.???
 
I agree with Ken Roach, there are several sources for DF1 drivers that are known to work. Writing your own, and with a time limit was a high risk job.

However from your last post I would suggest that the PLC is ok, the serial port and cable are ok. The problem lies in how you have structured your VB code.

Are you certain that there is no other device like a mouse driver also trying to use the serial port?
 
Got it!!!!

I have got to the root of the problem. The overflow was happening due to the 16th bit of a N9 file remaining high (this is considered a sign bit). :nodi: Hence the reply in the read was negative which was an overflow.
Eg.: if the PC was trying to read the N9:0 file and the N9:0/15 bit was high the overflow message was shown.
The message did not come for any other bit remaining high or low.

Thank you Philip And Ken. only when I decided to approach here did it click me to examine all aspects of my code.
One never knows what helps. :D :D :D !!!!!
 

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