Wiring to Non-standard RS-485 Device

[AdamFJungkunz]

I am designing an RS-485-based system for reading differential pressure from a pressure transducer. The attached diagram from the datasheet shows the pinout of the device, and the manufacturer confirmed the pinout this way:

Pin 1: Power +

Pin 2: Output +

Pin 3: Common (Also serial port common)

Pin 4: Power –

Pin 5: Tx

Pin 6: Rx

Obviously the pin out includes neither differential Tx and Rx pins nor Data (A)+ or Data (B)- pin designations, which seem to be the standard ways of wiring RS-485 devices together. (I have received the command list from the manufacturer, and it includes a command for setting the address of the slave device, which tells me that this is indeed an RS-422/RS-485 device.)

I'm planning to connect to the device with an NI 9871. I've read the 9871 Getting Started Guide (http://www.ni.com/pdf/manuals/372277g.pdf) and this KB (https://knowledge.ni.com/KnowledgeArticleDetails?id=kA00Z000000P7mNSAS&l=en-US), and I'm pretty sure that my best bet is to try wiring the device up in 2-wire mode. However, I'm not sure which pairs of pins to short together on the NI 9871 side to make it work. Do I need to short TX+ & RX+ together and TX- & RX- together or short TX- and RX- together and wire that to the common of the device? Has anyone seen this topology before?

[K C]

Hi,

Devices with a RS-485 interface are specified in a lot of ways. This one is new for me.

You are right you would expect a differential line. Tx and Rx on RS-485 makes no sense. Also it cannot be the Tx and Rx I/O of a RS-485 transceiver because there is no enable signal.

Can you measure the voltage of the Tx and Rx pin ?

Kees

[AdamFJungkunz · Accepted solution]

Kees,

Thanks for your reply. The manufacturer replied to me with this pin mapping:

NI 9871       P855 Pressure Transducer

Pin 1: GND  - P855 Pin 3

Pin 2: CTS+

Pin 3: RTS+

Pin 4: RXD+ - P855 Pin 5

Pin 5: RXD- - P855 Pin 6

Pin 6: CTS-

Pin 7: RTS-

Pin 8: TXD+ - P855 Pin 5

Pin 9: TXD- - P855 Pin 6

This matches the KB I listed above as the typical 2-wire RS-485 topology with the device's Pin 5 serving as the Data (A) + line and the device's Pin 6 serving as the Data (B) - line. That makes a lot more sense.