RS485 2 wire on NI 9871

[Steffen01]

Hello,

do I need a converter if I want to wire RS485 2 wire onto the NI 9871? The NI9871 says its RS485, so I just joined RXD-/TXD-  and RXD+/TXD+ and connected it to the - and + on the RS485 output. But in VISA test panel I don't get any read, just timeout. But Query returns 6 bytes for *IDN?\, the open ports get timeout on query

Cheers,

[lagelian]

Steffen01, 

The NI-9871 does support RS485, so a converter should not be necessary. Is there a timeout error code that you can provide? What software version are you using? 

Angela L. 

National Instruments

[Steffen01]

Hi Angela,

I get these 2 errors. I will just put a terminator resistor in, see if that helps. I read something, on 2 wire more its better, to put them in. Also there was something, connect TXD- to RXD+ and vise versa for 2 wire and then to - and + of the Modbus device

Cheers,

Steffen

[Steffen01]

[lagelian]

Thanks for the screenshots. I also did some digging through our past discussion forums and found one that was pretty similar. Have you looked through this yet? 

NI 9871 wiring to device with A and B terminals 

https://forums.ni.com/t5/Instrument-Control-GPIB-Serial/NI-9871-wiring-to-device-with-A-and-B-termin...

[Steffen01]

yes, connected 4 with 8 and 5 with 9, then to + and - of the modbus card, this one.

http://www.carel.com/bms-building-management-system-interface/-/journal_content/56_INSTANCE_i4q5KIML...

Put in a 100 Ohm resistor for terminator just then, see if that changes something. Its just 0.5m long cable.

Installed all the drivers on the cRio.

[JÞB]

 Modbus uses an RS 485 physical layer.  However, it has additional requirements for other layers.  Are you using NI-Xnet?

Modbus does not act like SCPI

[Steffen01]

http://www.carel.com/c.pco

its this one wit a modbus card, on 19200 Baud, nothing extra. Fot now just trying to see if I got the wiring right in MAX and VISA test panel.

Slave address is 001, not 01, but that should not make a difference, still just 01 in hex.

Configure Labview as Master, how? Is there options in MAX?

I just got an answer I need this one, a repeater. Or Can I set the 9871 somewhere, this channel is 2 wire, not 4? This only works in Labview, not in MAX? I found some examples there, half duplex.

http://www.bb-elec.com/Learning-Center/All-White-Papers/Serial/4-Wire-to-2-Wire-Connections.aspx

set windows for half duplex:

http://digital.ni.com/public.nsf/allkb/3EB322DAEAACB19D862571F500021C00

[Steffen01]

still no luck, but found this

http://digital.ni.com/public.nsf/allkb/1D516C10D7EDFAC6862571110073B8F4

https://www.ni.com/getting-started/set-up-hardware/instrument-control/drivers

will try tomorrow

suppose I have to set MAX to 2 wire.

atached some visa test, fro each register it returns its nuber times 256 as reading. But at least no timeouts