RS-232#
The legacy industrial UART. Same framing as UART but shifted onto bipolar voltages (±5 to ±15 V) and inverted, so the operator cannot tie an RS-232 line directly to an MCU pin without a level shifter. Still on factory-floor PLCs, network-gear console ports, and serial-attached scientific instruments.
Wires#
Signal |
DB9 pin (DTE) |
Direction |
Purpose |
|---|---|---|---|
|
3 |
Out |
Transmit data. |
|
2 |
In |
Receive data. |
|
5 |
Reference |
Signal ground. |
|
7 |
Out |
Request to send. |
|
8 |
In |
Clear to send. |
|
4 |
Out |
Data terminal ready. |
|
6 |
In |
Data set ready. |
|
1 |
In |
Data carrier detect. |
|
9 |
In |
Ring indicator. |
Wire format#
Framing is identical to UART (start, data, parity, stop), but the electrical layer is inverted and bipolar.
Mark (logical 1) is -3 to -15 V.
Space (logical 0) is +3 to +15 V.
Idle is mark (negative voltage).
Common baud rates match UART; the slow end (300, 1200, 2400, 9600) shows up more often than on bare-MCU UART.
A MAX232-family level shifter bridges between RS-232 voltages
and CMOS UART; without one, the operator’s USB-UART adapter does
not survive contact with an RS-232 port.
Pads#
DB9 is the dominant connector. RJ45 is the second variant, introduced by Cisco; the pinout is vendor-specific.
DTE (Data Terminal Equipment) is the host (laptop, PC).
DCE (Data Communications Equipment) is the modem or the device the operator is talking to.
A straight cable connects DTE to DCE; a null-modem cable swaps
TXD/RXDand other pairs so two DTEs can talk directly.
Tools#
Tool |
Effect |
|---|---|
USB-to-RS-232 adapters ( |
Provide |
|
Level shifter ICs; turn a 3.3 V UART into RS-232 and back. |
|
Same terminal emulators as UART. |
Multimeter |
Read line voltages before plugging in; mistakes here fry adapters. |
Vendor pinout sheet |
Cisco, HP, and Sun publish their own RJ45 console pinouts; the operator confirms before crimping. |