USB rs-485 pinouts

[Tolo-Tech]

I can't seem to get the USB to RS-485 Cable to work with Modbus RTU communication.  My system works with third party USB-rs485 converters but not with the NI cable.  I have installed the driver properly, and the communication settings are correct. 

On the DB9 connector I have Ground on pin 1, Data+ on 8, and Data- on pin 9.  Is this correct?  I should be able to communicate with half-duplex right?

Thanks for the help.

[SteveChandler]

Edit: Withdrawing my comment. For some reason I read RS232., I don't know about 485

[.aCe. · Accepted solution]

This may be of help: http://www.ni.com/pdf/manuals/371253c.pdf

[drjdpowell]

The link provided by .:aCe:. beats me to it, but for half duplex you have to connect both the transmit and receive pins to the two wires (you've just connected one set).

[LesPeters]

Does the RS485 device that you are trying to communicate with use the 2-wire mode or 4-wire mode?  The NI USB-485 converter comes wired for the 4-wire mode.  To use it in the 2-wire mode simply install a jumper between pins 4-8 (+) and 5-9 (-) on your DB9.  Connect (+), (-), and ground (pin-1) to your device.  If you have already done this make sure in MAX that you have configured your RS485 port for 2-wire auto mode.  Here is a link to a serial quick reference guide that I like to keep around (http://www.ni.com/pdf/manuals/371253c.pdf).

[Tolo-Tech]

OK, So I just hadn't read that manual closely enough.  Once I jumpered the pins 4-8, and 5-9, then set it to 2 wire Auto mode in MAX it worked great.   Thanks guys

[hybeijing007]

Dear LesPeters,

If i've a USB-to-RS485 cable, do i still need to figure out whether to communicate in 2-mode or 4-mode please?

regards,

Yi

[MCOTO]

HI Tolo-Tech,

Remember that the DEFAULT connection will be 4-wire. If your device requires a 2-wire connection please follow the steps shown in the link below.  Otherwise you don’t need to configure anything.

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

Regards,

[heroldw]

I tried this usb to rs485 it works without any jumper, the question is how is this cable different than the NI cable?

https://www.amazon.com/dp/B01MA1G70P

[pincpanter]

It's better not to use old threads for new questions (even worse, a thread that already has a solution).

Anyway, the fourth picture in the product page clearly show the pinout: 1TR+ 2TR- for 2 wire, 1T+ 2T- 3R+ 4R- for 4 wire, while the NI adapter uses pins 4, 5, 8, 9.

So what's the question exactly?