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Consider a RS422/RS485 receiver like the AM26LV32 or the DS26LV32. It is used to receive the signal of incremental encoders with differential outputs. Assume that those are using drivers such as the AM26LV31. There is only receiver on the bus, i.e., driver and receiver are at opposite ends of the cable.

Seemingly to facilitate the use of encoders without differential outputs, the schematic omits the termination typical for RS422/RS485. On the receiver side, there is no resistors between the positive and negative line of the differential pairs. Instead, the schematic has a 1kOhm pulldown and a 2kOhm pullup (to 5V) on both the positive and the negative line of each differential pair.

Datasheets typically recommend a termination resistor (200 Ohms or so) between the positive and the negative line of each differential pair. As far as I understand, that helps with reflections. Now this schematic omits it.

What will be the consequences of omitting the termination? What issues are likely to occur when attaching encoders which may reach switching rates as high as 30 MHz? You may assume that these encoders have differential outputs.

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    \$\begingroup\$ Link to the schematic of the development board that supposedly doesn't have terminations. \$\endgroup\$
    – Andy aka
    Commented Jun 20, 2020 at 10:45
  • \$\begingroup\$ Re " ... I cannot name the manufacturer or post the schematics, ..." -> that greatly reduces the ability of people to give a useful answer. If what you see if not std practice then knowing who is recommemnding non std practice MAY tell people much. \$\endgroup\$
    – Russell McMahon ♦
    Commented Jun 26, 2020 at 10:18
  • \$\begingroup\$ I described the schematic and clearly described the application of the circuit. What you need to know is already there. Who designed the circuit does not matter. Whether a circuit is good or bad, what are its advantages or disadvantages, doesn't depend on who designed it. It is entirely determined by the target application and the environment, both of which I have described. \$\endgroup\$
    – Sven
    Commented Jun 27, 2020 at 13:16

1 Answer 1

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You can have a bus where a single RS422 transmitter drives many receivers, so the termination must be only at the last node, and other nodes must not have a termination. Omitting the termination from the board gives the opportunity to wire the boards on the bus as you like and terminate the bus separately. It also allows the termination resistor to be selected based on bus impedance, e.g. 100 ohm or 120 ohm. If the bus is short and data rate is low it may work fine without termination, so you have the option also not to terminate. In your case there environment is noisy so termination is a good idea to keep low and balanced bus impedance.

The pull-ups and downs are there to keep a default receiver state if it is disconnected from bus, sometimes this feature is built in to chips.

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  • \$\begingroup\$ In this particular case, there is only one receiver on the bus. The driver is at one end of the cable, the receive is at the other end of the cable. \$\endgroup\$
    – Sven
    Commented Jun 20, 2020 at 13:00
  • \$\begingroup\$ The pull-ups and -downs are the same for both the negative and the positive lines of the differential pairs (as I wrote above). So if nothing is connected, the positive and negatives lines of the differential pair are at an equal voltage. However, the receivers required a differential voltage of 200mV. \$\endgroup\$
    – Sven
    Commented Jun 20, 2020 at 13:03
  • \$\begingroup\$ Well to understad better why the professional module does what it does, we'd need to know what module it is, for what purpose it is made for, the user/installer manual how to connect it to something, and the schematics. \$\endgroup\$
    – Justme
    Commented Jun 20, 2020 at 13:08
  • \$\begingroup\$ The hardware is for receiving the signals of an incremental encoder with differential outputs. I cannot name the manufacturer or post the schematics, unfortunately. \$\endgroup\$
    – Sven
    Commented Jun 20, 2020 at 13:15
  • \$\begingroup\$ Interfacing with such encoders should be quite standardized and there should be plenty of application notes from encoder and receiver manufacturers to know the best practices when dealing with them. \$\endgroup\$
    – Justme
    Commented Jun 20, 2020 at 13:22

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