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Pinout lookup
Connector pinouts drawn the way the connector looks, with voltage levels and a multimeter check for each. Verify against the datasheet for your exact part and board revision before applying power.
RS-232 DB-9
Looking at the MALE DB-9 on a PC (DTE), solder side opposite. Pins 1-5 top row, 6-9 bottom row.
Levels: Mark ≈ -3 to -15 V, space ≈ +3 to +15 V. Idle line sits at a NEGATIVE voltage.
- Power
- Ground
- Signal
- GPIO
- No connect
1
DCD
Data carrier detect (in)
2
RXD
Receive data (in)
3
TXD
Transmit data (out)
4
DTR
Data terminal ready (out)
5
GND
Signal ground
6
DSR
Data set ready (in)
7
RTS
Request to send (out)
8
CTS
Clear to send (in)
9
RI
Ring indicator (in)
Notes & checks
- Two DTE devices need a null-modem cable: swap 2↔3, 7↔8, 4↔6, keep 5↔5.
- A female DB-9 has the numbering mirrored; do not read a plug's pins off the socket drawing.
- Never connect RS-232 directly to a 3.3 V/5 V UART pin. Use a MAX3232-class level shifter.
- To check: with the port idle, measure pin 3 (TXD) to pin 5 (GND). A real RS-232 port reads about -5 to -12 V.