Instrument Control (GPIB, Serial, VISA, IVI)

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Why is the idle voltage of pxi8431-8 board card bus 0v in 2-wire auto mode?

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In general, the serial communication idle level should be high level.Why is the idle voltage of pxi8431-8 board card bus 0v in 2-wire auto mode?

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Message 1 of 10
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Hi,

 

Maybe you could read this post first. The answer is there.

 

Kees

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Message 2 of 10
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Hi Kees

Thank you for your anwser!
When using 2-wire auto mode, I set the following items:

1. the short between RX & TX.

2. The VCC pin is not extracted on the board card, so I can't do pull up and down resistance.

3.Since my external device has requirements on the idle level of serial port, I am very concerned about the idle level of 485.

So I want to know whether to set up the 485 board card port of 8431/8 in the device manager? Or what external auxiliary circuit should be added? To keep the idle level high.


Looking forward to your reply.

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Message 3 of 10
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Hi,

 

As you probably know the RS-485 is a bi-directional bus. So it is only active if one of the connected devices is in transmit mode. The only way, I know, to 'force' the lines into a know (fail-safe) state is to use a active terminator at one side.

You won't see many (master) devices with this option.

 

Kees

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Message 4 of 10
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Hi Kees

Thank you for your anwser!

 

I connected two ports of pxi8431/8, COM1 and COM2, and used COM1 to send data and COM2 to receive data.

Both ports were set in 2-wire auto mode. I shorted the TX+ and Rx+ ,TX- and Rx - for COM1. Also, shorted the TX+ and Rx+ ,TX- and Rx - for COM2 . Then connect the two ports.

Use oscilloscope to observe the data on the line. When data transmission. Idle level is 0V.

 

Looking forward to your reply!

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Message 5 of 10
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I think you cannot say: 'In general, the serial communication...'  while you are talking about RS-485.

Why should it be high ??

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Message 6 of 10
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HI Kees

Thanks!

In the attachment, you can see that the idle bit in the serial port data is high, while the idle bit I tested is 0V. Although the idle bit is 0V and can communicate normally. one of my devices requires a high idle level for 485 communication, otherwise it will cause other problems.

 

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Message 7 of 10
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Hi,

 

I you refer to a two state line it has to be in one state (high or low) for RS-485 there is this grey area with no communication where the line is undefined.

Although we always use fail-safe terminators for our devices it is not absolutely necessary. With the use of defined lines the is no receiver that will receive anything during the idle period. Also if bus repeaters are used they will stay in one state.

If the line is not idle a receiver could receive something. The chance that it will receive a valid byte is close to zero. The chance that it will receive a valid frame (of a protocol) is zero !! 

I never saw the requirement of a high idle level. 

 

Still if you want a solution you could solve this with a additional external power supply to power the fail-safe terminator. But you will need a 0V connection to the devices !!

 

Kees

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Message 8 of 10
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Hi ,Kees

 

Is there any software such as NI MAX that can set the idle level of pxi8431-8 board card to be high in the 2-wire auto mode?Or how can it be set in software?

 

Thank  you!

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Message 9 of 10
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No, not as far as I know.

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Message 10 of 10
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