First Character in RS-485 message

[wa5ycg]

Friends:

I am doing some RS-485 communications between two PC's running my LV program.  I am using a third party USB/RS-485 converter in four-wire mode.  There are only two devices in the network and I can see the data going back and forth at all data rates using good and bad media, terminated and not.  My problem is that the first character in the data stream is a hex 00, followed by my message in good order.  And VISA reports a -1073807298 error "Could not perform operation (VISA Read) because of I/O error.  The third party converter manufacturer (big name) reports that since it works find with a terminal emulation program (which does not report non-printable characters), then there is nothing wrong with the hardware.

Does anyone out there know why an RS-485 transmitter circuit could be introducing a false start bit when turing on?  Any other clues as to how to mask this?

Steve Hope

CompuFlow Solutions

[K C]

Hi Steve,

Let me guess, it's a FTDI device.

I had both problems  with FTDI devices.

The I/O error was solved using the VISA 'Set I/O buffer size' function for Rx and Tx. This was years ago. Later I discovered that this problem was solved. New driver, different computer ??

The zero's  I see in an application with a 9-bit protocol. I just delete the zero's because the message newer begins with zero. I never could figure out what was the cause of the three (in my case) leading zero's which are not present at the interface side.

Kees.

[wa5ycg]

No.  It is a BlackBox device.  Their tech support simply says it is not their problem because of terminal emulator works without a problem.  But I am pretty sure that I am seeing a start bit about the time that the transmitter turns on and that is why I am getting the zero.  I guess that I just need to try someone else's hardware.  I can not find anything to stop the LV error or the extra charcter.

S

[billko]

---

@wa5ycg wrote:

No.  It is a BlackBox device.  Their tech support simply says it is not their problem because of terminal emulator works without a problem.  But I am pretty sure that I am seeing a start bit about the time that the transmitter turns on and that is why I am getting the zero.  I guess that I just need to try someone else's hardware.  I can not find anything to stop the LV error or the extra charcter.

S

---

Is it possible to just flush the buffer before you attempt your first I/O?

[wa5ycg]

I have tried doing the VISA buffer size flush/change after initialization suggested in other posts with no luck.  There is also a VISA CLR just before the VISA read that is causing the -1073807298 error accompanied by the extra character.  I am just stuck!

Oh, just tried to loop back using the same VI and it works perfectly.  Then did the loopback with the long (6 feet) of cable, still works fine.  It screws up when I use two PC's and to USB to 485 converters.