Connecting to RXD and TXD pins

[Tom_SAVU]

I have to connect a stepper motor having RS-485 interface to my PCI-8433/2 board. The motor has a DB-9 connector with two data pins marked "RS485A (+)" and "RS485B (-)".

Other pins are used for the power supply (2 pins), for connecting sensors (3 pins) and for relays (2 pins).

In the DB-9 pin description provided in the "NISerial.chm" file, there are four pins marked RXD+, RXD-, TXD+ AND TXD-.

Question 1: to which pins on the PCI-8433/2 board do I have to connect the two data pins of my stepper motor ?

Question 2: do I have to make also other connections, like pin 1 of the PCI-8433/2 to the GND pin used for supplying power for the motor ?

[mhead]

Connect the RDX(+) to the TXD(+), connect RDX(-) to TXD(-).  If a terminating resistor is required, it shorts out these two lines.  Some specify 200 ohm for this, but I use 500 ohm due to my product limitations. In the computer device manager you will also need to select a 2 wire mode.  I use 2 wire auto.

RS485/RS422 and RS232 have a lot in common.  I am currently using NI 4 port 485/422 cards to talk both 2 wire RS485 and RS232 serial by using resistors in the connector.  The language and voltage levels are in practice the same.   

[Andy F.]

I would assume that the communication with the device is going to be one way (from the computer to the stepper motor) so I would recommend connecting the RS485A (+) and RS485B (-) pins to pins 8 and 9 which are the TXD+ and TXD- lines.  It may not be required, but connecting the GND pin of the stepper motor to the GND pin (which is pin 1) is probably a good idea. 

Hope this helps!

Andy F. 

[mhead]

Two wire RS485 is bidirectional, but not at the same time.  It is called simplex (as opposed to duplex).  It is also multi-drop, in that multiple devices can be connected to these two wires.  Software handshaking handles transmit/receive as well as addressing control.  USB (universal Serial Bus) does the same thing.  The way this is done is by using high impedence tri-state devices for both transmit and receive.  Self test is easier, because the device talks to itself on the same wires--and there is one of the problems--software (or hardware) must mask out  transmissions with a delay or some other means.

[Tom_SAVU]

Thank you very much, I will try this solution.

[Tom_SAVU]

First thanks were for Andy.

Thanks to you also mhead, I will try also your solution