How do I communicate with a RS-232 device with my NI PCI-485 board?

[MBEer]

I need to connect an instrument that uses the RS-232 protocol (i.e. RXD, TXD, GND, DTR, CTS) to a National Instruments' PCI-485 board.  Please let me know how I can accomplish this.

[Henrik_Volkers]

Do you really need Hardware flow control? ( DTR and CTS ?)  

If not and you have a free port, you can use a 'simple' level shifter .

If you need to address it in a RS485 network ...???? maybe its also possible

Here is a link to a company that maybe can help you (I use their adapters and they do the job fine):

 http://www.wut.de

[JoshuaP]

The best solution would be to get a NI PCI-232 board. Anytime you try converting a 485 to 232 or a 232 to 485 you will run into problems. There are several companies that sell converters, but most of them will only support a subset of the 232/485 control lines. If you look through the forum you will notice that most people end up getting framing errors when they take this approach. Especially if you are wanting to use some of the flow control, hand shaking lines, ring indicator, or modem control lines.

This is just my opinion.

-Josh

[MBEer]

Hi Henrik,

     thanks for your note.  I don't need to use hardware flow control.  So what are the connections that I need to make on the PCI-485 side to make this work?

Thanks again,

Rajeev

[Henrik_Volkers]

You will need a free RS485 port to use a simple converter since you can't use the bus adressing.

I also run into framing errors in the past, however I never had these problems with the wut devices (using external power supply, seems that the 'selfpowered' ones are to slow...) 

I used the 8600 in 4 wire automatic modus.

[mhead]

I have a 4 port 485/422 card (National Instruments, of course) that I use to talk with 2 wire 485 devices and also 232 devices.  All flow control is in software.  All it take is several resistors.  For the 485 devices, on the NI DB9 connector, pin 4 and 8 are shorted and run to the 485-B terminal; pin 9 and 5 are shorted and run to the A terminal, and I have a terminating resistor of 500 ohms across the two.

For the 232 devices, on the NI side DB9 pin 9 goes to pin 3 on the RS232 connector; pin 8 goes through a 100 ohm resistor to a common connection, pin 4 also goes through a 100 ohm resistor to the common. and pin 1 goes directly to the common. This common then attaches to pin 5 on the RS232 side. On the NI side, pin 5 attaches to a 200 ohm resistor which then attaches to pin 2 on the RS232 side.  Now on the 232 side, short out pin 6 to pin 7.  The last issue is the shield.  It is to be grounded on only one side, not both.  You must check the attaching connector on the NI side, because some of the NI cables are grounded at the computer, some are not.  The common above is a ground reference, so it can be used to ground the shield.  But make sure the connector on the other end is not grounded when it is attached to the device you are talking to.