Hi,

I'm having a few issues with the RS485 CB1241 Module, I need to communicate to some scales via simple ASCii protocol using the PtP function blocks. I have an array of chars which I send via the send_ptp function block, but on the  receiving end ( current USB - RS485 adapter and my PC ) I get random data almost as if it was the wrong encoding or something.

I have tried both ptp librarys ( SEND_PTP and Send_P2P )  with the same results. I have tried using the port configuration function blocks and without. I have tried different RS485 to USB adapters with the same results. I have read through the S7-1200 Manual and it says you can use the CB1241 and RS485 with the ptp

I have just noticed on the data sheet it mentions  (  — ASCII Yes;  Available as library function  ) where do I find this library function? I have searched all through the siemens website.

I have attached some pic of my program sections and data being sent along with the received data.

Note: If I send ascii data to the PLC from the PC, the received data is all random and weird also

I will head back to site tomorrow and have another crack at it, hopefully someone can shed some light before I head back there.

Thanks!

Hi @qwazee 

No I'm not after Modbus ASCii, just plain serial ASCii

How do I swap the byte order of a CHAR in Tia Portal if I need to? I take it you are referring to MSB and LSB? 

I did take a quick look at the HEX output and it was hard to tell what was going on, but none of the HEX values were the value of the ASCii characters. 

I did make sure there were no headers as you can see in the port config block the XON and XOFF are blank.

Unfortunately I don't have a CB1241 in my workshop to test this. Can I do this via s7 simulator? 

No, unfortunately there is no way to simulate this. Since the CB1241 has no function onsite, it might as well have gone home with you (with needed permissions).

The instructions you showed looks different than what I am used to. Which version of the instructions are you using? ..and which Step7 version? Mention also which software you are using to send/receive ASCII.

Start small. Send a message with the same character. Both ways. Record and have a look at the hex. Repeat the tests a few times. There should at least be a repeated pattern. Else there are stuff added to the message string that is going to cause problems.

I assume that you have the correct terminating resistors in place and using the correct shielded cables.

Having x2 S7-1200 would have been ideal for workshop tests. If you are on a good footing with your Siemens supplier, ask if they can lend some demo kit.

Hi @Duris_I

I have had the scales functioning connected directly to my PC and send the correct ASCii via the terminal. So I can confirm I have the correct format for the scales. I confirmed the weight request polling and Tare and Zero commands. 

I have tried non optimized data blocks also but I had the same result. I have tried both HW cfg and FB cfg and I get the same result with both. 

What is the best way to log the DONE, NDR, ERROR and STATUS? Just assign to tag or FIFO buffer? 

Yesterday I tried just simple ASCII such as a single letter. If I send a simple character 'A' (#41) I get an  _  line (#5F) at the other end. No matter what I try it's always incorrect. Even I change the char to byte I get exact same result

I have monitored the HEX on serial monitor which is 5F and compared that to ASCii table. I have also converted the HEX to LSB and MSB and compared that to ASCii table also, but nothing is correct. 

Could I have a faulty CB1241 Module? I have used 2x different RS458-USB adapters to confirm on was not faulty, have used 3x serial monitor programs to confirm it was not serial monitor. Have gone over all com port settings again and again

Thanks!

Hi @Duris_I

I figured it out thanks to logic analyzer and oscilloscope... A of my device now goes into B of CB1241 and vice versa and now it works as expected... I suspect I have a module that is labeled wrong. Or correct me if that's a siemens thing? Since when with RS485 does A got to B and B to A? I have done plenty in the past and this would be a first for me. 

I know with UART/RS232 TX goes to RX and RX to TX. But have never come across this for RS485? Almost makes me think the module is labeled wrong or something wrong with the hardware. 

NOTE: A from scales to A to my RS485-USB Converter and vice versa for B works perfectly fine. I even looked at all the CB1241 wiring diagrams and termination resistors etc and everything online shows A - A and B - B

If anyone comes across this and having issues with CB1241 just give swapping A and B around on the CB1241 and save a lot of time trying to debug bad frames with logic analyzer and oscilloscope  which I ended up doing. It wasn't until I swapped channels on the analyzer software and it displayed correct data, so I swapped physical wiring on CB1241 and it's all correct now! 

I would like to say thanks to Duris_I and qwazee for there help.

It is actually a thing. One of the members discussed the reason why there is a mix-up with the to RS485 connections. The short version an incorrect design layout was published and used by some manufacturers as standard.

From my side I have not had any issues with connecting various manufacturers VSDs onto Siemens' CB1241.

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