08-29-2019 05:05 PM
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?
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08-30-2019 01:36 AM
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
08-30-2019 08:11 AM
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.