How much power can be drawn from the RS-232 bus?
Drawing power from the RS-232 signal lines was not part of the original 1960s bus design. It is possible to draw several milliamps because:
- RS-232 has relatively high voltage levels. RS-232 signals driven by typical RS-232 transceivers are 5V or more.
- A voltage level of 0V is not defined or not used. Logic ‘0’ or the idle state is indicated by +5 to 25V; logic ‘1’ is indicated by -5 to -25V.
The ability to power anything by drawing power from the RS-232 signal lines is an accident of creation. Drawing power from the RS-232 signal lines is dependent on the design details of the source device, and would lead to unreliable results when source devices change.
Another complication is that some manufacturers will drive a bus with logic level signals and RS-232 timing, marking their device as RS-232 compatible.
What is RS-232 handshaking?
Handshaking, or flow control, is a way of slowing down the data so that a slower receiver can keep up with a faster transmitter. For example, if a receiver’s buffer is full it can use flow control to stop the transmitter temporarily.
Handshaking can be done in hardware over RTS and CTS signal lines. Using hardware handshaking requires a five wire interface: ground, receive data, transmit data, RTS and CTS.
RS-232 handshaking can also be done in software by sending control characters. In this scheme, the XON / XOFF control characters are inserted to the data stream. Software handshaking can be done using only a three wire bus: ground, receive data and transmit data.
When do I need a null modem cable?
The easiest way to tell is if pin 2 is an input on both devices or pin 2 is an output on both devices, you need a null modem cable. If you are connecting inputs to outputs and vice versa, then the two devices will be able to talk to each other with a straight cable.
If you don’t know the input / output status for both devices, hopefully you know whether each is a DCE or DTE. If you have two of the same, they need a null modem cable to talk to each other. If you have one of each, they can talk to each other on a straight cable.
The most difficult part to remember is what to do when you only know the input / output status for one device and the equipment type for the other. DTEs have an input on pin 2. DCEs have an output on pin 2. Now you can use the rule above to avoid having two devices using the same line as an output.
Who controls the RS-232 specification?
The current revision of RS-232 is TIA-232-F Interface Between Data Terminal Equipment and Data Circuit-Terminating Equipment Employing Serial Binary Data Interchange, issued by the Telecommunications Industry Association in 1997. A commonly used revision is the Electronics Industry Association’s (EIA) standard RS-232-C of 1969. RS-232 was originally written as a “Recommended Standard (RS)” by the EIA in 1962.
How does RS-232 differ from RS-422 and RS-485?
RS-232 is a point-to-point connection between two devices. RS-422 can be a multi-drop connection with up to 10 receivers, but a maximum of 1 transmitter. RS-485 is a multi-point bus which may have over 30 devices communicating on it.
RS-232 signals are single-ended or ground referenced. RS-422 and RS-485 signals occur as differential pairs.
RS-232 has been used extensively in nearly every conceivable application since its inception in 1962. RS-485 was approved as a standard in 1998. Some OEMs have added this option to simplify wiring or to overcome issues of electrical noise coupled onto the bus. RS-422 is not used as commonly as the other two. Some specialized applications may use RS-422 drive circuitry to drive signals with RS-232 timing and protocol.
Why won’t RS-232 die?
- It’s common. Everyone knows about it and is comfortable with it.
- It can be implemented inexpensively.
- Relatively large voltage swings make it more immune to noise than something like the USB bus. Although it is less immune to noise than differential buses like RS-485 and Ethernet. The data may be read from the stick by any computer with a USB port.