RS-485: PC can send/receive data from PCI card, but LabVIEW can only send to card.

Question

I have a master/slave setup with a 2-wire RS-485 connection via a StarTech PCI2S485 card  http://www.startech.com/product/PCI2S485-2-Port-PCI-RS-422-485-Card-with-DB9.  A Vista PC running LabVIEW 2010 is acting as the master to 6 slave devices.  I am running a driver VI supplied by the company that made the slaves. 
I've used a terminal program called Termite to monitor the ports during operation and have concluded that both the PC and the slaves are sending correct data packets (i.e. correct protocol and reasonable data).  Termite indicates that the slave's response is being received by the PCI card, but the problem is that LabVIEW simply isn't reading the response from the card.  Each time the driver VI is run it returns a "timeout waiting for slave to respond" error.  
The VI and slaves I am using are designed for both RS-232 and 485 communication.  I've verified the VI to run in 232 mode on a different COM port.  I also wrote a simple VI using the VISA functions to test the ports on the PCI card, but I've had the same results.  Hardware connections have been looked over carefully, but the fact that the card actually sends a correct message and receives a correct message indicates a different problem.
Any idea why LabVIEW can talk to the PCI card but can't hear it?  Is there anything I can do about this, or does LabVIEW generally not like StarTech's PCI2S485 cards?
Thanks,
Tim

Answer 1

Tim359 ha scritto:
I have a master/slave setup with a 2-wire RS-485 connection via a StarTech PCI2S485 card  http://www.startech.com/product/PCI2S485-2-Port-PCI-RS-422-485-Card-with-DB9.  A Vista PC running LabVIEW 2010 is acting as the master to 6 slave devices.  I am running a driver VI supplied by the company that made the slaves. 
I've used a terminal program called Termite to monitor the ports during operation and have concluded that both the PC and the slaves are sending correct data packets (i.e. correct protocol and reasonable data).  Termite indicates that the slave's response is being received by the PCI card, but the problem is that LabVIEW simply isn't reading the response from the card.  Each time the driver VI is run it returns a "timeout waiting for slave to respond" error.  
The VI and slaves I am using are designed for both RS-232 and 485 communication.  I've verified the VI to run in 232 mode on a different COM port.  I also wrote a simple VI using the VISA functions to test the ports on the PCI card, but I've had the same results.  Hardware connections have been looked over carefully, but the fact that the card actually sends a correct message and receives a correct message indicates a different problem.
Any idea why LabVIEW can talk to the PCI card but can't hear it?  Is there anything I can do about this, or does LabVIEW generally not like StarTech's PCI2S485 cards?
Thanks,
Tim
RS232 interface consist of 2 indipendent communication crossed lines between 2 devices (9DSub connector: Pin 2-RX  Pin 3-TX pin 5-Gnd)
RS485 interfaceis a bus, shared between 2 or more devices. (like ethernet or CAN Bus)
This means that an incorrect use of the bus can lead to a "packet collision" if 2 or more devices try to send a message on the bus at the same time or at least their messages, partially "overlap"
About 10 years ago I have used a PCI 2 RS485 port: I remember  that I have to change the status of RTS serial control line to switch between transmit and receive mode.
This can be done using the serial VISA property:
Serial Settings --> Modem Line Settings --> DTR State
or using the VI
<LabVIEW dir>\vi.lib\Instr\_sersup.llb\serial line ctrl.vi   (LV 8.6.1)
The result was that the slave device connected, send me the response after few nanosec. causing the lost of the answer.
because LabVIEW use more than some  nanosec. to change RTS property after sending message on the 485 bus calling a VISA Write.
So for me there was 2 possibilities: 
- use 2 RS485 port (1 for transmit 1 for receive echo transmit included)
 - use this converter  
         http://www.advantech.com/products/ADAM-4521/mod_2E78D425-8B08-43F6-81B0-1B924E53E075.aspx
 this converter, after sending a message on the bus, electronically switchs back to receive mode without losing a byte.
But the question is :
what does it means
"RS-485 mode supports Auto Transceiver turn around (ATTA)"
of your card?
bye