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I have a doubt, when I have a circuit that has a driver for a serial interface, such as a TTL-to-RS485 or TTL-to-422 buffer. The typical situation I'm in is that this chip is connected on one side to an MCU and on the other side to a connector on the edge of the board. The question is, is it better to have the buffer as close to the MCU as possible or as close to the connector as possible? Reasoning in the first case (as close as possible to the MCU) I have the shortest possible 3.3V TTL line, so I minimize the noise on the MCU-BUFFER side. While in the second case (as close as possible to the connector) I favor the signals coming out of the connector. Which situation is the most advisable.

PS: the board is general purpose, so the environment is not defined a priori.

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  • \$\begingroup\$ That depends on many factors which are not known, such as total distance between connector and MCU, and data rate. And it might differ between RS-485 and RS-422, because RS-422 must start from a single transmitter driving a bus, while you can have tens of RS-485 transceivers anywhere on a bus. Anything is doable and depending on what are your limitations one might be better than the other. There is no one answer which is generally better. \$\endgroup\$
    – Justme
    Mar 13 at 12:02

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If you follow good design practices (and under "normal" conditions), it does not make big difference where you place the transceiver.

Good design practices would be to have impedance controlled signal lines from the transceiver to the connector and to have good signal integrity on the CMOS/TTL lines from MCU to transmitter. And of course you would need to have little cross-coupling from/to other signals on the board, etc.

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    \$\begingroup\$ It also depends how the transceiver is wired to the connector. Any wiring between connector and transceiver is a stub if you make it a stub, so in which case good design rules would say to minimize the stub length. It can also be designed to not be a stub, if you have separate in and out connectors (e.g. DMX-512). Which means the rules are different between a multidrop RS-485 bus and point-to-point RS-422 bus. \$\endgroup\$
    – Justme
    Mar 13 at 13:31

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