10-11-2011 03:48 PM
That sounds reasonably well to run the write commands twice then. EEPROMs and other devices have similar safety mechanisms as well.
The resistors I spoke of weren't actually the bias resistors but instead terminating resistors. The Wikipedia entry has a nice picture of 680 ohm bias resistors with 120 ohm terminating resistors in the RS485 Overview section but lacks information in other sections. Actual values will depend on the load of the transmission line
The purpose of the bias resistors is to keep the positive line at a greater potential than the negative line when no devices are turned on or connected. Without bias resistors and without connected devices, the lines could float to different voltages and give strange results when reading the line. Check the documentation for the device you are using to turn on/off bias resistors and see what it is actually doing.
The purpose of the terminating resistors are to keep reflections to a minimum (what you may be seeing and calling corruption). This Maxim IC guideline does a fair job analyzing the purpose of terminating resistors, look closely at Figure 10. Basically the value of the resistor should be the same as (or close to) the characteristic impedance of the transmission line used. There isn't a single value for all situations.
Adding in an 8th controller may create just enough load on the line to make it unuseable without the resistors. Other references to RS485 mention you can have up to 32 devices on the line at fairly large distances.
10-13-2011 02:07 PM
Anthony,
I switched by PC back to 32bit mode in Windows 7, and everything is working great. I don't know why, but 64bit mode causes issues. Do you have any thoughts or idesas on why there is such a difference in behaviors between the two?
10-14-2011 02:08 PM
That's very interesting.
When you say that you switched back to 32bit mode, do you mean 32bit LabVIEW with 64bit Windows or 32bit LabVIEW with 32bit Windows?
If you switch the Windows OS there might be some difference in the device drivers
10-18-2011 11:28 AM
Anthony,
I switched back to 32-bit LV with 32-bit Windows. But your question makes me realize that I didn't look for different drivers before, and now I should if I wish to go back to 64-bit Windows.