Serial, RS232
Overview
Communication as defined in the RS232 standard is an asynchronous serial communication method. The word serial means, that the information is sent one bit at a time. Asynchronous tells us that the information is not sent in predefined time slots. Data transfer can start at any given time and it is the task of the receiver to detect when a message starts and ends.
Voltages
The signal level of the RS232 signal can have two states. A high bit, or mark state is identified by a negative voltage and a low bit or space state uses a positive value. This might be a bit confusing, because in normal circumstances, high logical values are defined by high voltages also. The voltage limitations are shown below.
| Level | Transmitter capable (V) | Receiver capable (V) |
|---|---|---|
| Space state (0) | +5 Vdc up to +15 Vdc | +3 Vdc up to +25 Vdc |
| Mark state (1) | -5 Vdc up to -15 Vdc | -3 Vdc up to -25 Vdc |
| Undefined | - | -3Vdc up to +3Vdc |
Pinouts
| View - looking into Male connector | View - looking into Female connector |
 |  |
| Pin No. | Name | Dir | Notes/Description |
|---|---|---|---|
| 1 | DCD | IN | Data Carrier Detect. Raised by DCE when modem synchronized. |
| 2 | RD | IN | Receive Data (a.k.a RxD, Rx). Arriving data from DCE. |
| 3 | TD | OUT | Transmit Data (a.k.a TxD, Tx). Sending data from DTE. |
| 4 | DTR | OUT | Data Terminal Ready. Raised by DTE when powered on. In auto-answer mode raised only when RI arrives from DCE. |
| 5 | SGND | - | Ground |
| 6 | DSR | IN | Data Set Ready. Raised by DCE to indicate ready. |
| 7 | RTS | OUT | Request To Send. Raised by DTE when it wishes to send. Expects CTS from DCE. |
| 8 | CTS | IN | Clear To Send. Raised by DCE in response to RTS from DTE. |
| 9 | RI | IN | Ring Indicator. Set when incoming ring detected - used for auto-answer application. DTE raised DTR to answer. |
Achievable distances
The maximum distance that RS323 can go varies depending on Bit Rate and also the capacitance of the wire itself. Texas Instruments states that RS 232 maximum cable length is 50 feet, or the cable length equal to a capacitance of 2500 pF. Depending on the cable such as Category 5 Twisted pair the maximum achievable distance at 19200 is 157 feet.
here is a quick chart showing theoretical limits as various data rates for RS232 cable length according to Texas Instruments
| Baud rate | Maximum cable length (ft) |
| 19200 | 50 |
| 9600 | 500 |
| 4800 | 1000 |
| 2400 | 3000 |