مخطط الأطراف

مخطط أطراف وحدة مرسل-مستقبل RS-485 من نوع MAX485 (لوحة توسعة 8 أطراف)

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الأطراف

• RO — receiver output, data coming off the bus (push-pull at VCC, so 5V logic — divide it down before feeding a 3.3V MCU)
• RE — receiver enable, ACTIVE LOW (LOW = listening; HIGH = receiver muted)
• DE — driver enable, active high (HIGH = transmitting). Tie DE to RE and drive both from one GPIO: LOW = receive, HIGH = transmit
• DI — driver input, the TX line from your MCU (idle HIGH; the module does not echo your own bytes back on RO while DE is high — wait for the last byte to finish shifting out before dropping DE or you truncate the frame)
• VCC — 5V only, 4.75–5.25V. The plain MAX485 is NOT a 3.3V part: for a 3.3V MCU use a MAX3485 / SP3485 board instead of feeding this one 3.3V
• A — non-inverting bus line (also silkscreened Y, D+ or A+). Needs a 120Ω termination resistor to B at each of the two physical ends of the bus only
• B — inverting bus line (also silkscreened Z, D− or B−). Idle line state is A above B; a long idle bus needs fail-safe bias resistors (or a transceiver with built-in fail-safe) or the receiver reads noise as data
• GND — must be common with the MCU, and the two ends of the RS-485 cable need a ground/reference conductor between them: A and B alone are not enough over any real distance

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