MODBUS_RTU_SET_CUSTO(3)

✨ modbus_rtu_set_custom_rts ✨

okay bestie, so this one lives in the libmodbus Manual, section 3, and it's literally all about making the RTS pin do your bidding. 💅

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🌸 NAME 🌸

modbus_rtu_set_custom_rts — set a function to be used for custom RTS implementation

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🎀 SYNOPSIS 🎀

int modbus_rtu_set_custom_rts(modbus_t *ctx, void (set_rts) (modbus_t ctx, int on))

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🦋 DESCRIPTION 🦋

so the modbus_rtu_set_custom_rts() function shall set a custom function to be called when the RTS pin is to be set before and after a transmission. by default this is set to an internal function that toggles the RTS pin using an ioctl call. no cap, that's the factory vibe. ✨

⚠️ note that this function adheres to the RTS mode, the values MODBUS_RTU_RTS_UP or MODBUS_RTU_RTS_DOWN must be used for the function to be called. like, your custom function literally will not get called without one of those, so don't come crying to me. 😭

and this function can only be used with a context using a RTU backend. that's the whole rule. 🫶

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💖 RETURN VALUE 💖

the modbus_rtu_set_custom_rts() function shall return 0 if successful. 🎉

otherwise it shall return -1 and set errno to one of the values defined below. 🙈

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🌈 ERRORS 🌈

• EINVAL — the libmodbus backend is not RTU. 💀

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👑 AUTHORS 👑

Jimmy Bergström <jimmy@ekontroll.com> 🧚

the libmodbus documentation was written by Stéphane Raimbault <stephane.raimbault@gmail.com> 🌸

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and that's the whole page, bestie — libmodbus v3.1.6, dated 11/17/2025. go forth and toggle those pins responsibly 💅✨🦋