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I am interested in a datasheet's schematic for my own PCB design of a similar system. In my system, I am receiving an RS-422 differential line, and would simply be tempted to put a 100 Ohm terminating resistor between the differential data lines and then send them directly to the line receiver.

However, in addition to this terminating resistor (82.5 Ohms in their case), they included 147 Ohm resistors on each line to ground. What is the purpose of these?

Moreover, they seem to have put a low pass filter with a 8.25Kohm and 2.2pF capacitor, which corresponds to a cut-off frequency of 8.7MHz which is reasonable for the use case but they connected the capacitor to +5V instead of GND which I would have assumed. Why did they connect these capacitors to +5V instead of GND?

Here is their circuit

enter image description here

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  • \$\begingroup\$ My guess is that REF and ~REF are relative to 5V, e.g. they might be designed to take levels like REF being 0V and ~REF being +10V, or vice versa. The polarized capacitors still make no sense in this scenario, so I would probably approach this schematic with a healthy amount of scepticism. \$\endgroup\$ Dec 3, 2023 at 1:26
  • \$\begingroup\$ Anyway, nothing can really be said about this, and you're not supplying a link to that datasheet, although you're quoting a schematic from it. Please fix that to a) adhere to site rules and to good practice in general and b) to allow us understand what's happening here. \$\endgroup\$ Dec 3, 2023 at 1:27
  • \$\begingroup\$ Thanks Marcus. Unfortunately, I do not have the datasheet and I cannot find it, I found this schematic on a website which didn't mention its origin. I think you're right regarding the filtering and the reference being used. If I want to filter an RS-422 line on my pcb, should I do it before or after the line receiver? \$\endgroup\$
    – Tommy95
    Dec 3, 2023 at 2:02
  • \$\begingroup\$ Can you then at least link to that website? Every bit of content helps, and come on, cutting your sources really is not only a rule we have here, but also a matter of good style. \$\endgroup\$ Dec 3, 2023 at 2:26
  • \$\begingroup\$ Sounds like something that can be ably ignored. The attenuation also reduces maximum distance and noise margin, which might not matter in some situations, but doesn't seem like a great idea. \$\endgroup\$ Dec 3, 2023 at 3:03

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In a multi-node network, the RS422 drivers each have tri-state outputs that can go into a high-impedance state to allow other drivers to drive the bus. When all of the nodes are in this state, the lines can float, possibly out of the common-mode range of the receivers. The resistance to ground is meant to prevent this.

Since RS422 was designed for long cable lengths, the two ends of a long cable often each have their own local, independent power supply even though they have connected grounds. In your circuit, the 2.2 pF capacitors are connected to the local power supply voltage on the "receive" end, which may be separate from the "drive" end of the cable.

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  • \$\begingroup\$ Small correction: RS-422 is always-on, RS-485 is multi-master. \$\endgroup\$ Dec 3, 2023 at 4:21
  • \$\begingroup\$ No, that would be a large correction : RS-422 does not support sharing the bus with multiple transmitters. It only allows one transmitter at bus end, one set of termination on the other end, and up to ten receivers. \$\endgroup\$
    – Justme
    Dec 3, 2023 at 10:44
  • \$\begingroup\$ Thanks John. In my case, both devices (transmitter and receiver) work with the same 12V power supply present at the RS422 receiver side, carried through a 3 meters-long cable to the RS422 transmitter side and regulated to 5V independantly on both sides. \$\endgroup\$
    – Tommy95
    Dec 3, 2023 at 12:01
  • \$\begingroup\$ Is there any potential benefit in adding these 2.2pF capacitors? Perhaps it would be more relevant to pair them to GND instead of +5V. \$\endgroup\$
    – Tommy95
    Dec 3, 2023 at 12:25
  • \$\begingroup\$ Hi Tommy. The advantage would be for a receiver that had some type of power supply/common mode rejection issue. I would install them if your specific receiver recommends them. If not, it would probably be just as good to have the capacitors go to ground. You probably don't need them at all in your application. \$\endgroup\$ Dec 4, 2023 at 15:31

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