Is there any problem with the 485 board card of NI's pxi8431/8?

[runningtree]

The idle level of output in 2-wire auto mode is 0v, shouldn't it be logical high +5V? Check some data that  there is no bias resistance, but the bias resistance  how to add,  no one can explain in detail. How to solve this problem, hope answer!

[K C]

Hi,

If you want to sure that a RS-485 network is always working you must use terminator resistors at both ends. (typically 120 IOhm)

If you want to force the level in the idle state you can use a fail-safe terminator at one end. In that case you need access to the Vcc and Gnd.

See attached pdf example.

There should also be a 0V (Gnd not earth) connection between devices.

Kees

[runningtree]

Hi Kees

Thank you for your reply. I would like to confirm the following two questions. First, is there no bias resistance on the 485 board card of pxi8431/8? Second, you said that VCC and GND should be used. The 68pin port on my board card is connected to the serial port cable with product number: 185845-01, so VCC cannot be obtained.Could you please tell me in detail how to connect the wires if you want to add bias resistance?

[K C]

In the ideal case you would add the fail-safe terminator as in my diagram. But if you don't have access to the Vcc (and Gnd) of the circuit just use a single resistor terminator at both side. (just like the one at the right in my drawing)

If devices have a terminator resistor on board it will be switchable. As far as I know NI does use terminator resistors on their devices.

[runningtree]

First of all, thank you，Kees

I’m sorry that I am a freshman. I have checked a lot of materials and found that Bias resistors are not available on eight or 16 port PXI rs-485 hardware. I know what you said about terminal resistance, but what I want to do now is to make the idle level at a high level, such as 5v. How do you add this bias resistor so that the idle level is pulled up to a high level when you can't get a VCC?

[K C]

If you don't have access to the Vcc of the interface (or a connected device) it will be rather difficult to connect a fail-safe terminator.

You could add an external power supply for this but it is not a nice solution.

Your network will also work without the fail-safe terminator, but than your network will float if it's idle.

The problem with RS-485 network is that there is a lot of documentation around. But I found that there is also a lot of wrong information.

Look at the IC manufactures sites of RS-485 transceiver IC's (like TI and others) for information. Most of them have good documentation about RS-485 networks.

But still if manufacturers of interface (like NI) don't care about terminators, wiring Vcc and Gnd to the connector, using the Gnd connection between devices, you have to find the solution that suites your situation the best.

I use a lot of RS-485 devices and interfaces and every time it is different (no Gnd connection, no terminators, A and B line reverse, etc.) and every time I have to find a solution to get it working.

Hope you find a solution in your situation. Just let me know if I can help you further.

Kees