Module 03 ~40 min Interactive

Connectors & Pinouts

A wrong pin is the most common reason a "finished" board won't talk. Click through the interactive DE-9 to learn every RS232 signal, wire up RS485 two-wire and RS422 four-wire terminal blocks, and — most importantly — learn the A/B naming trap that has cost engineers whole afternoons.

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The 9-pin D-sub (correctly DE-9, commonly called DB9) is the classic RS232 connector. This is the DTE (computer/male) view. Click any pin to see its function.

Click a pin to see its signal name and function.
Only three pins are strictly required for basic communication: TX (3), RX (2), and GND (5). The rest are hardware flow-control and modem lines that many modern links simply don't use.

Unlike RS232's fixed D-sub, RS485/RS422 field wiring has no single mandated connector. Screw terminal blocks dominate industrial gear; RJ45 is common in structured cabling. What matters is the signal set, not the housing.

Signal RS485 (2-wire, half-duplex) RS422 / RS485 4-wire (full-duplex)
Data pair 1 A / B (also D+ / D−) TX+ / TX− (a.k.a. Y / Z)
Data pair 2 — (shared, half-duplex) RX+ / RX− (a.k.a. A / B)
Ground / reference GND / signal common GND / signal common
Shield Chassis (one end) Chassis (one end)
Always run a ground/common wire alongside the data pair(s). The differential receiver still needs both wires to stay within its common-mode window (Module 2) — a shared reference keeps them there.

Here's the one that bites everyone. The RS485 standard defines the pins as A and B, where A is the inverting and B is the non-inverting line. But many transceiver vendors and connector silkscreens label them the opposite way, or use D+/D− with no consistent mapping to A/B.

TIA-485 standard

A = inverting (−)
B = non-inverting (+)
Idle (mark): B > A

Many datasheets / silkscreens

Label "A" as the + and "B" as the − — the reverse. D+/D− add more confusion.

The rule: ignore the letters. Wire by polarity — non-inverting driver output to non-inverting receiver input — and verify against the transceiver datasheet, never the silkscreen label. Good news: swapping A/B causes no damage; you just get no communication. Try the swap when a new bus won't link.

Answer to reveal instant feedback.