I want to use RS422 to communicate two MCUs. In a serial protocol, RX from MCU1 goes with TX from MCU2 and viceversa. RS422 is differencial, so I don't really know how to manage this. Where do I connect TX_P, TX_N, RX_P and RX_N in the microcontroller's pins? In my MCU I only have SCI_TX and SCI_RX. And how, after that, do I connect one MCU which another? How can I interface both? I need some isolation between both microcontrollers too. I don't understand the schema.
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asked Nov 3, 2023 at 13:13
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\$\begingroup\$ What data rate, how much isolation? Generally you would use dedicated hardware to interface between MCU tx/rx pins and 422 wires. Start with your isolation requirement, as that will drive your choice of interface components. 'Some' is not a specification. Why RS-422? \$\endgroup\$– Neil_UKCommented Nov 3, 2023 at 13:31
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\$\begingroup\$ So, the order would be MCU - Driver - ISO or MCU - ISO - Driver? The isolation is not high voltage, maybe 50V. I was using MAX22025 but is half-duplex, I want that but with the four wires (dual). \$\endgroup\$– Developing ElectronicsCommented Nov 15, 2023 at 13:14
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RS422 is a hardware layer standard enabled by placing a RS422 transceiver (for example the classic MAX485) outside the microcontroller UART peripheral (also known as SCI in the NXP/Motorola world). You will need at least one per microcontroller.
Isolation is a separate matter. The best solution is usually to buy a separate digital isolator and place it outside the transceiver - since this gives you more BoM options. The alternative is to pick a transceiver with galvanic isolation built-in, but these are poorly standardized and so you end up "marrying" the silicon vendor.
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1\$\begingroup\$ (Although in the year 2023 when our clueless governments merrily have allowed all silicon vendors to merge and gain complete monopolies in their respective markets, designing electronics = marrying the silicon vendor. Not long ago I would perhaps have picked an Analog Devices part and specified Maxim as 2nd source... or even pitted them against each other to see where I could get the best price for equivalent parts.) \$\endgroup\$– LundinCommented Nov 3, 2023 at 13:51
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\$\begingroup\$ But could I connect the four signals (RT+, RX-, TX+, TX-) to a digital isolator without affecting the data? \$\endgroup\$ Commented Nov 15, 2023 at 13:12
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\$\begingroup\$ @DevelopingElectronics You have to check rise/fall times in case you are using very high baudrates. You'd place the isolator on the logic side of the transceiver in most cases. There are also various niche digital isolators designed for specific buses in mind, here's an article: analog.com/en/analog-dialogue/articles/… \$\endgroup\$– LundinCommented Nov 16, 2023 at 7:21