USB 485 connection for single device

[Laucalvino]

Hello,

I need to make a very simple connection but even though I have looking at forums and manuals for days I still cannot figure out how to do it. Maybe someone can help me, here it is:

I need to control an actuator that is "compatible with RS485". The manufacturer gave me the pinout of the controller cable with these pins: +5V, TRx+, TRx- and Signal ground. I also have an schematic were I see that TRx+ and TRx- are wired to a SN75176 bus transceiver (pins A and B) so I am confident that I can connect my brand new NI USB-485.

The only problem I have is that I do not know how to build my female connector! I have to control only one actuator, and the cable is short (no need for terminations or anything of that).

Does anyone knows how to do it? I feel very dumb holding three cables in my hand from the actuator and not knowing how to plug them into the RS485 

Thank you!

[K C]

Hi,

Yes, many times a RS-485 connection is confusing to a lot of people. It is not difficult, but you have to know what you are doing.

First you can have a 2-wire or 4-wire connection. This is already misty in your post. If you say TRx+ and TRX- you talk about a 4-wire connection (also referred to as RS-422). But this also should include RX+ and RX-. But the SN75176 is a 2-wire RS-485 transceiver.

Your USB-RS485 device can handle both.

First you have to find out is your RS-485 device is a 2 or 4-wire device. If the manufacturer only specifies the TX signals this could mean that the device is only transmitting data. Is this correct ?

Do you have a specification of your device that I can look into ?

[Laucalvino]

Hello again,

I only have four pins (TRx+, TRx-, Ground Signal, +5V) and the specification is for RS485. So based on these and knowing they use the SN75176, I would assume this is a 2 wire connection. I have attached the manual.

The device (a linear actuator) receives data, not just send. It receives positions for example and sends out alarms.

Here is the only circuit I've got from the serial connection, and then how everything is supposed to be connected. Please note that the things in the blue circle come if I purchase the software from the company (which I do not need at all).

Thank you for the help!

[K C · Accepted solution]

Based on this information I would suggest that you connect the + to pin 4 and 8 of the USB-RS485 interface and the - to pin 5 and 9. Connect the 0V to pin 1.

To make it complete (specially if you use a long cable) you should add a active terminator at one side (as in your picture) and a passive terminator at the other side.

The values in the picture are far too high. It should be 2x 390 and 1x 150 Ohm. The passive terminator at the other side should be 120Ohm.

Kees

[Laucalvino]

Thank you! 

I am trying to try this. I thought I could be able to use the USB 485 cable with an example for my presser written in C directly (without Labview), but it does not work. I will write my VI and test the connection then. I will let you know as soon as it works 

[Laucalvino]

I made it work very easily directly from C (I forgot to configure the port as 2 wire).

Thank you for the help once again