Serial Communications Standards
 
Most communication is "serial" - a single electrical, optical or wireless channel carries a signal as a stream of bits. 

Over the short distances within chips or equipment cabinets it can make sense for signals to be parallel - arithmetic and logical operations are often simpler and quicker this way. Over the longer distances of local and wide-area networks it is usually better to marshall the parallel bit-patterns from device registers into a serial link. 

There are many varieties of serial communication, differing in data transmission speed, intended geographic range, number and type of clients and protocol complexity.
 
 
WAN

The wide area links used by telecoms companies tend to be of the "OCn" or T1/E1 "WAN" types. The electronics for this sort of link can look sophisticated and the lines do carry high bit rates, but the actual signal may just be a simple time-division multiplex. A design presumption underlying these systems was that they would mainly carry telephone conversations, but they can actually handle the dense traffic of Internet backbones quite efficiently.
 
LAN

Local area links include 

USB (so local its confined to the desk)
Firewire - about 10 times faster than USB and better suited to video
Ethernet and WiFi - a general purpose network normally confined to a campus

These Local Area Networks are local links use complex protocols to share the cable between competing uses and make things easier for non technical users. The link may be simple, but it uses a lot of support software.
 

About 
Serial & Parallel
RS232
RS422
RS423
RS449
RS485
RS530
RS644
Index- Serial 
USB

Some communication needs don't fit the LAN or WAN category very well. A group of communications standards are often known simply as "serial". This group is mainly made up of the "RS" recommended standards devised by the US EIA (Electronics Industries Association).
 
RS232 in OutlineEIA Standard RS232 "Interface Between Data Terminal Equipment and Data Communication Equipment Employing Serial Binary Data Interchange."

The most widely used computer industry point-to-point link is usually called RS232.

The RS232 standard originated as a means to connect computers to modems - as a LAN to WAN interface rather than a communications link. RS232 is still used in this role to connect modems and ISDN adapters.

However RS232  also used to attach mice, printers, PCs, musical and industrial instruments to computers.

RS232 is a very long-lived standard; however it is rather limited:
 
Line lengths are not generally supposed to exceed 15 metres. Much longer lines often do work but not at high speed.
Data rate for most equipment is rather under 40,000 bits per second – less than 1/200th the speed of an Ethernet. With FIFO buffering UARTs RS232 serial lines can achieve speeds up to 1 million bits per second. Since other network standards deliver up to a billion bits per second RS232s electrical properties remain a limitation.
Setup of RS232 links can be complicated- a lot of hardware and software signals can be involved so there is a lot of room for incompatibility. There are few automatic setup mechanisms

It may seem surprising that RS232 is still used, but it does offer some advantages:
 
Relative simplicity. RS232 is complex for end-users, but relatively straight-forward for technicians. Developing hardware and writing a program to receive RS232 data can be simpler than the same task using Ethernet or Token Ring
Low cost. RS232 does not demand support from a protocol stack running on a powerful processor - in fact basic RS232 links do not strictly even need a processor.
Ubiquity. A great deal of equipment that does not require high speed communication has an RS232 interface. This means there is a presumption in favour of RS232

To overcome RS232s many limitations the EIA has produced several standards intended to provide improved serial communication:

  • EIA Standard RS422 “Electrical Characteristics of Balanced Voltage Digital Interface Circuits”
  • EIA Standard RS423 “Electrical Characteristics of Unbalanced Voltage Digital Interface Circuits”
  • EIA Standard RS449 “General Purpose 37 Position and 9 Position Interface for Data Terminal Equipment and Data Circuit Terminating Equipment Employing Serial Binary Data Interchange”
etc

These standards have not enjoyed the same wide take-up as RS232. One problem is market inertia, there is a huge installed population of RS232 equipment, so new devices need the ability to connect to it. RS423 is quite often used instead of RS232. The two standards can work together, so a manufacturer sacrifices very little by using RS423.

The main problem is that the newer EIA standards were obsolescent by the time they were introduced. If medium-distance, high-speed communication is needed the natural answer is Ethernet.

Ethernet is still complex to implement, so there was a market gap for simple, low-cost communications devices. Universal Serial Bus (USB) succesfully aimed at this market.

A decade ago system designers wanted to avoid the complexity of Ethernet, so new serial communications standards may have seemed an interesting prospect. However Ethernet hit a “sweet spot” in the market, so the cost of single chip Ethernet adapter circuits fell to equal or better serial communications and performance was better.

Ethernet chip-sets generally offer better support for high-speed serial data links. As serial transmission speeds are increased the support circuitry for the transmitter and receiver need to grow in complexity, otherwise the main processor in host equipment has to support the communications. Recent PCs have 16550 FIFO buffering UARTS to cut down on the number of interrupts to the main processor caused by the serial port. The 16560 has a deeper buffer and should further improve performance.
 
 
CharacteristicsRS232RS422RS423RS485RS646/LVDS
Transmission TypeSingle EndedDifferentialSingle EndedDifferentialDifferential
Transmission
Mode
SimplexSimplexSimplexHalf DuplexSimplex
TopologyPoint to PointMultidropPoint to PointMultipointMultidrop
Maximum Data Rate20Kbps nominal10Mbps10Mbps35Mbps655Mbps
Maximum Cable Length15m nominal1200m1200m1200m30m
Voltage Levels
Logic 1 Mark
Logic 0 Space
-5V to -15V
+5v to +15V
-2V to -6V
+2V to +6V
-3.6V to -6V
+3.6V to +6V
-1.5V to -6V
+1.5V to +6V
0.247V
Minimum Receiver Voltage-3V
+3V
-0.2V
+0.2V
-0.2V
+0.2V
-0.2V
+0.2V
+100mV
-100mV
Maximum Driver Slew Rate30V us92.3V/ns