9 bit multidrop, doable?

[jeffh12345]

Hi All,

I am getting ready to do RS485 communications using a form of 9 bit multi-drop protocol on my next project

(the first byte has the 9th bit set, the rest of the packet does not)

Now, I know that you can get the ninth bit by playing with parity bit (MARK or SPACE)

The problem is do I want to try: has anyone had success with this?

A few years ago, we did a proof of concept on this in C++: it failed

There was too much latency in the windows operating system: sometimes it worked, sometimes not

My baud is about 57.6 K

Any hardware or software recommends would be appreciated

Thanks,

Jeff

[Adam G]

Hello Jeff,

All of our 485 devices should be able to work with the multi drop configuration. Here is a link to a knowledgebase that talks about the using multi drop with 485. You can see all of the 485 devices that we offer. For the software to control it I would recommend using LabVIEW. We also have a knowledgebase that tells how to do 9 bit communication in LabVIEW. I do not have any personal experience doing 9 bit multi drop though, does anyone else? Please let me know if you have anymore questions. Have a great day!

Best Regards,

Adam G  

[Wayne.C]

Jeff,

I've not done any 9 bit protocol but I have fooled around with programatically asserting and deasserting the DTR line on 232 ports.  I found the USB-Serial adapters don't work very well for stuff like this due to overhead/delays in the USB driver layer.  Takes forever to assert/deassert a control line or change the parity mode on the fly.  Best bet is to use something like a PCI or PCIe card thats plugged directly into the pc backplane.

[K C]

Hi Jeff,

I use the 9th bit protocol with LabVIEW and it works fine.

Sending: change the parity in MARK and SPACE like you said.

Receiving: In the VISA config file disable the replacement character (DisableErrorReplacement=1)

                Ignore error after receive data (parity error)

Let me know if this works for you.

You also can use hardware that support this. I use this solution for a test. 

Kees