RS-232 is a digital data transmission protocol with origins dating back to the 1960’s. It was designed as a communication protocol to talk between DTE (data terminal equipment) and DCE (data communication equipment). It is closely related to the UART communication protocol provided by most microcontrollers, except that RS-232 uses higher-voltage signalling whilst UART is restricted between
VCC of the microcontroller and
|Num. Wires (excl. GND)||2 (TX/RX) or 4 (TX/RX and RTS/CTS)|
|OSI Layers||Layers 1 (physical) and 2 (data link)|
RS-232 is commonly used today for a variety of different purposes in embedded systems, incl industrial equipment, test and measurement equipment. RS-232 ports are no longer available on most desktop computers (and certainly not on laptops), but USB-to-RS232 adapters are cheap, popular and easy to use with almost any operating system. Microcontroller UART peripherals are commonly connected to a UART-to-RS232 converter to send data that is going off-board.
- RS-232C (or just shortened to RS-232)
- EIA-232-D (1987)
- EIA/TIA-232-E (1991)
The flow control is a way of detecting when the receiver or transmitter is ready to accept or send new data. The UART protocol provides a few (all optional) methods for flow control:
- Hardware RTS/CTS lines
- Software XON/XOFF flow control
This is done by various additional connections to the standard transmit, receive and ground wires. The two most common are CTS and RTS. Typically, a micro-controller may have one or two UART peripherals that support this feature, while the rest are just basic non-flow control UART’s.
A small amount of power can be extracted from the RTS and CTS lines for powering low-power devices.
The following tables lists all of the flow control signals (as well as the data signals), with respect to the device in question. Matching signals are grouped together.
|Signal||Port Type||Description||DB-25 Pin||DE-9 Pin|
|DTR (Data Terminal Ready)||Output||DTE drives this to indicate to the DCE that it is present.||5||4|
|DCD (Data Carrier Detect)||Input||DCE drives this when it is connected to the telephone line.||8||1|
|RI (Ring Indicator)||Input||The DCE drives this when it has detected a phone call. Note that there is no matching signal going the other way.||22||9|
|RTS (Request To Send)||Output||DTE drives this to tell the DCE to get ready to receive data.||4||7|
|CTS (Clear To Send)||Input||Driven by the DCE when it is ready to accept data.||5||8|
|TxD (Data Transmit)||Output||The DTE sends data to the DCE over this line.||2||3|
|RxD (Data Receive)||Input||The DCE sends data to the DTE over this line.||3||2|
|Common Ground (GND)||n/a||The common ground for all signals.||7||5|
|Protective Ground (PG)||n/a||The protective ground. This is usually just connected up to the common ground on the PCB.||1||n/a|
Rule-of-thumb: Most RS-232 serial interfaces with a male 9 or 25 pin connector are DTE’s, most with a female 9 or 25 pin connector are DCE’s.
Most often, manufacturers label the UART pins as DTE’s. In this case, you have to swap all connections with their matching line. So TxD of device 1 is connected to RxD of device 2, RxD of device 1 is connected to TxD of device 2, RTS of device 1 is connected to CTS of device 2, e.tc.
15m or less
Do you need a higher-level communication protocol that works over a UART connection? See the SerialFiller library on GitHub (written in C++). SerialFiller uses a publish/subscribe mechanism and works well on point-to-point serial connections such as UART.
The pinout of a typical Analog Devices RS-232 to UART transceiver is shown below.
Another example of a RS-232 to CMOS UART converter is the MAX3221IDBE4. It supports an auto-shutdown feature based on the voltage-level of the receiving RS-232 line.
The Maxim MAX3227 is a popular RS-232 line transceiver. It contains 1 driver and 1 receiver as is designed to be driven from CMOS signals (e.g. connected to a microcontroller UART)1. Texas Instruments produces a drop-in replacement part for the MAX3227, the MAX3227CDBR (the CDBR standing for commercial temp. range, tape and reeled)2.
Cheap Discrete-Part RS-232 To TTL Converter
A RS-232 to TTL logic-level converter can be made out of a few discrete components. The schematic shown below uses some clever circuitry, including a charge-pump like circuit, to generate the negative voltage required for RS-232 transmission back to the computer.
RS-232 DE-9 To RJ-45 Cables
Many routers contain a RJ-45 socket that carry a RS-232 bus. They are designed so you can plug in a DE-9 to RJ-45 cable and connect the router to a PC for configuration. One example of a cable is the Tripp Lite RJ45 to DB9F Cisco Serial Console Port Rollover Cable, as shown in the below image:
Maxim (2011, Feb). MAX3224–MAX3227/MAX3244/MAX3245: 1µA Supply Current, 1Mbps, 3.0V to 5.5V, RS-232 Transceivers with AutoShutdown Plus (datasheet). Retrieved 2022-03-02, from https://datasheets.maximintegrated.com/en/ds/MAX3224-MAX3245.pdf. ↩︎
Texas Instruments (2006, Feb). AX3227: 3V to 5.5V Single-channel RS-232 Line Driver/Receiver With ±15kV ESD Protection (datasheet). Retrieved 2022-03-02, from https://www.ti.com/lit/ds/symlink/max3227.pdf. ↩︎
Tripp Lite. RJ45 to DB9F Cisco Serial Console Port Rollover Cable, 6 ft. (1.83 m). Retrieved 2022-05-17, from https://assets.tripplite.com/product-pdfs/en/p430006.pdf. ↩︎
Juniper (2021, Nov 13). MX2020 Universal Routing Platform Hardware Guide: RJ-45 to DB-9 Serial Port Adapter Pinout Information. Retrieved 2022-05-17, from https://www.juniper.net/documentation/us/en/hardware/mx2020/srx4600/topics/concept/port-rj45-db9-adapter-pinout.html. ↩︎
U.S. Converters LLC. Making a DB to RJ45 adapter. Retrieved 2022-05-17, from https://www.usconverters.com/downloads/support/db9_rj45_assembeling_guide.pdf. ↩︎
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