Fundamentals of RS–232 Serial Communications
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04-10-2010, 10:20 AM
Due to it’s relative simplicity and low hardware overhead (as compared to parallel interfacing), serial communications is used extensively within the electronics industry. Today, the most popular serial communications standard in use is certainly the EIA/TIA–232–E specification. This standard, which has been developed by the Electronic Industry Association and the Telecommunications Industry Association (EIA/TIA), is more popularly referred to simply as “RS–232” where “RS” stands for “recommended standard”. In recent years, this suffix has been replaced with “EIA/TIA” to help identify the source of the standard. This paper will use the common notation of “RS–232” in its discussion of the topic. The official name of the EIA/TIA–232–E standard is “Interface Between Data Terminal Equipment and Data Circuit–Termination Equipment Employing Serial Binary Data Interchange”. Although the name may sound intimidating, the standard is simply concerned with serial data communication between a host system (Data Terminal Equipment, or “DTE”) and a peripheral system (Data Circuit–Terminating Equipment, or “DCE”). The EIA/TIA–232–E standard which was introduced in 1962 has been updated four times since its introduction in order to better meet the needs of serial communication applications. The letter “E” in the standard’s name indicates that this is the fifth revision of the standard.
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Why Serial Communications?
Serial communication is the most simplistic form of communication between two devices.
It’s pretty intuitive once you see the pattern.
It’s what started networking!
So What is RS-232?
RS-232 is a standard by which two serial devices communicate:
The connection must be no longer than 50 feet.
Transmission voltages are –15V and +15V.
It is designed around transmission of characters (of 7 bits of length).
One important aspect of RS-232 is that it is an asynchronous form of communication.
Asynchronous communication is important because it is efficient; if no data needs to be sent, the connection is “idle.” No additional CPU overhead is required for an idle serial line.
RS-232 is a little non-intunitave at first.
Logical 1 is –15VDC.
Logical 0 is +15VDC.
When the connection is idle, the hardware ties the connection to logical 1.
How Can You Transmit Data?
RS-232 communication is dependent on a set timing speed at which both pieces of hardware communicate. In other words, the hardware knows how long a bit should be high or low.
RS-232 also specifies the use of “start” and “stop” bits.
Sending One Character
Every time a character is sent, the same communication occurs:
1. Start bit sent.
2. Seven data bits sent.
3. Stop bit sent.
This communication is dependent on the fact that both devices are sampling the bits at the same rate!
Ok, So What’s the Start Bit?
The start bit is a logical 0 sent on the line to tell the other device to start sampling.
Remember, the logical 0 is +15VDC.
And the Stop Bit?
The stop bit is a logical 1. –15VDC.
A stop bit is always sent (per RS-232 standards).
A Sample Transmission
Common Serial Settings
Most settings are read in the following form:
Bits per second
Number of data bits
Number of Stop bits
Most everything comes out of the box with 9600,8,none,1. Including most Internet related devices like routers.
Another common (old-school) setting is 9600,7,even,2.
Line Sampling & Framing
RS-232 hardware samples the line multiple times during a single bit transmission.
If the samples do not all have the same voltage, a framing error occurs.
A framing error should only occur if one device is sending faster than the other device is set to receive.
An intentional frame error can be caused by sending a BREAK.
EIA RS-232 Serial Interface Standard
Basic 232 Specifications
Maximum Data Rate = 19,200 bps
Maximum Cable Length = 50 feet
Maximum Capacitance/foot = 50 pF
|Vmax| = ±25V
Negative Logic: logic 1 = -12V, logic-0 = +12V (typical)
2 Standard Connectors and Pinouts
DB9 is the 9-pin connector (DB stands for Data Bus)
DB25 is the 25-pin connector
*NOTE: DB25 is a general connector, NOT an RS-232 Only; PC parallel ports commonly use the DB25 Also!
RS-232 DB25 Pin Out
RS-232 DB9 Pin Out
RS232 Pins (cont’d)
Full Duplex Transmission
Full duplex transmission (FDX) occurs when data is transmitted (or can be transmitted) simultaneously by both devices. Special wiring is needed for FDX.
The RS-232 specification denotes usage of a 25 pin cable, where each pin has a specific usage.
However, most devices never need to use all of the pins, so the cabling requirements for specific devices may vary.
Many common serial devices (modems for example), use a 9 pin serial connection.
The two different connectors are associated with two major types of hardware
The Computer Terminal Equipment (DTE) and the Data Terminal Equipment (DCE).
For ease-of-use, a computer will transmit on pin 2 and receive on pin 3 (the DTE, remember).
Vice versa: a modem will transmit on pin 3, and receive on pin 2 (for the DCE).
For people familiar with modem communications, there is a speed limitation associated with the transmission.
56k (56 kilobit) analog modems are pretty much the fastest analog modems that consumers are going to see. This limitation is due to telephone systems, not the computer systems.
However, serial communications between devices also has its own speed barrier.
RS-232 was designed with the understanding that the analog world is far from perfect.
Digital is fast, analog is slow. RS-232 is analog, therefore is it slow (in computing terms).
Why Is It Slow?
Dt exists. The change is not instantaneous.
Sampling does not occur immediately, so it must wait Dt+t0
Cable length increases delay.
Signal noise is bad. It is caused by a variety of sources, all of which lead to lower speeds and less reliable transmission.
Shannon’s Theorem shows that the maximum transmission rate of a voice call (analog) is ~30,000 bps (30kbps).