RS-422 line driver to NI digital input termination

[Todd Worstell]

I am purchasing an incremental encoder that has an RS-422 line driver ouput.  This is a differential signal and requires transmission line termination.  There are many way to terminate this signal and it depends on the characteristics of the device, in this case an NI 9411 digital input.  However, I can not find much information on the electrical characteristics of the 9411.  What is its input impedance?  Does it need pull up/down resistors?  Does it need a termination resistor and of what value for 422. etc.  Basically, I am trying to come up with a circuit design appropriate for an 9411 connected to an 422 source.  I am looking at a quadrature encoder differential line driver becasue the encoder will be about 100 ft from the 9411 and I am worried about induced noise, voltage drop, reflections, etc on the transmission line.  Any suggestion?

[CKap]

Hello Todd,

Are you planning to use this module in a CompactRIO Chassis or a CompactDAQ Chassis?

Page 10 of the operating instructions for 9411 illustrates how you would connect a differential encode. Based on that diagram, I believe that you would not need to worry about the input impedance or the pull-up or pull-down resistors. I am not sure about the termination resistance though. Are you planning to power the encoder from a external power supply? If that the case then I would suggest that you consider possible grounding issues also.

Regards,

Chetan K

Application Engineering

National Instruments

[Todd Worstell]

I am using a CompactRIO chassis.  I did look at page 10 that shows how to hook up differential signals.  But is this differential TTL or differential RS-422 or are they the same thing?  I will be powering the enconder via the RIO system.

[Todd Worstell]

Forgot something............I was also wondering if I need a RS-422 receiver device before going into the RIO system. 

I am thinking that I can just take the encoder output, which is a RS-422 line driver, and wire it directly into the RIO system.  I have less than 100 ft from encoder to RIO.  I think I may have to worry about termination/pullup/etc if I have any problems. 

[Buechsenschuetz]

To connect a RS422 differential output to a TTL single-ended input, in most cases it is sufficient to connect just one of the differential lines to the input and provide common ground of the input device and the encoder. Each line of the differential output will swing between 0 and (almost) 5V, and that's what a TTL compatible input needs.

TTL input termination impedance is everything but well defined, since it will be different depending on what state the input is in. Anyhow, if you use just one of the differential output lines, this is just a working set up and not the proper one, so you probably don't have to care about termination.

The proper way is to use a level converter, such as the Am26LS32. This IC will convert differential signals (fed by both lines) to proper TTL level. You can add a termination resistor of 120 ohms in case there are any problems with signal ringing.

[CKap]

Just to add to the previous post. Keep in mind that you will have to write your own LabVIEW FPGA code to implement this protocol.

Regards,

Chetan K

AE-NIC

[Buechsenschuetz]

Of course, when connecting a RS422 output to a single-ended input device, you will loose some of the benefits of the RS422 standard. One important point is noise immunity - if spikes hit the (usually twisted) wire pair they will cause spikes of same polarity which will be ignored to a certain degree by a differential line receiver. Also, take care of proper GND connection. A differential connection is much more immune against GND level shifting which might occur along long GND lines between devices.

As mentioned by other posters, of course you will have to write your own code to decode the encoder signals. NI's motion controller boards have the firmware to decode quadrature encoder signals but I think using such a board for decoding a single encoder signal would be much overrated.

[Todd Worstell]

Thanks to everyone for input on this issue.  I plan to purchase a quadrature encoder Koyo TRDA-2E with TTL line driver output (RS-422 AM26L31) to replace my current quad encoder with standard TTL output.  I already have a working FPGA program to decode the quad signal.  I will start out with the simplest circuit I can implement - no termination, bias or pull-up resistors, just direct connections into differential digital input of NI-9411 module.  I will report back with results.

[CKap]

Great Todd. Let us know how it goes.

Regards,

Chetan K

AE-NIC