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Serial Communications - RS232
RS232 was designed to connect computers to modems but it has been a popular option for connecting terminals and printers as well. The interface specification creates a load of control signals which some modems do need, but most printers do not.
The RS232 interface became popular outside it's original sphere because it can move a page of text-only material over the sort of distance found within buildings in under 2 seconds. If users want to connect a remote office they just use a pair of modems. RS232 is a very flexible technology:
- Fairly long lines are possible. A line length of 15 metres is within spec, but cables up to 50 metres can work.
- Cables can be simple – if the printer supports X-on / X-off then just 3 wires are needed (ground and two signals)
- Maximum speed is about 150 kilobytes per second over a short cable a metre or so long. Sustainable speeds are about 10 kilobytes per second over long cables. At this speed a page of densely printed text takes just half a second or so to move, but a page of graphics may take two minutes or more. RS232 can be a bottleneck.
- RS232 often allows a trade-off between line length and speed. Short lines (in an office) can work quickly. Long lines (to the warehouse) work more slowly.
At one time RS232 was built in on all the more expensive printers and was an option on lower-priced machines.
In the 1980s and early 1990s green screen terminals became the standard way to interact with computers and they used RS232 so it was natural that the same standard was used for printers.
Few manufacturers now see much use for such a slow standard so the option is being dropped. On the odd occasion this is a nuisance because some industrial equipment like barcode readers, automatic weighing machines and suchlike do use RS232 and don't really have a use for graphics. External RS232 to parallel converters can be used - but presumably supplies of these will dry up.
The basic problem for RS232 is that modern printers are used in a new way - they are asked to produce graphics. Printing grapical information equires a 10 to 100 fold increase in the speed of communications. RS232 cannot deliver this sort of increase in data communication speeds.
In most cases there is no need for RS232. There are three main alternatives:
USB (Universal Serial Bus) is the most common on small printers. USB is actually a bus so the printer becomes closely integrated with the computer it is connected to. USB cables are a maximum of 5 metres long, although using active extenders 5 extensions can be plugged together making a bus 30 metres long.
Parallel-port extenders can operate up to a kilometre if long lines are required. Parallel extenders will usually work over cheap Cat-5 cable or even phone wire and operation is transparent - there is nothing to set up beyond the hardware. Some parallel extenders are bidirectional - but read the specification because some are not.
Ethernet is by far the best option in most cases. Ethernet can operate up to seven successive wire segments 100 metres long and print-servers can cost under £50. The protocols used are usually TCP/IP so traffic can in principle be routed anywhere on the Internet (indeed firewalls need to make sure it is not). Ethernet does need setting up - most print servers can emulate LPD and a couple of other protocols. All recent computer operating systems support TCP/IP so connecting via Ethernet is rarely a big technical problem.
Ethernet -TCP/IP printer interfaces and servers are by far the best way to connect a printer because it can easily be shared with the est of the network. Even if print-sharing isn't wanted the remote diagnostic and control abilities are generally useful - its much easier to Ping a printer to find out if it's working that to traipse 300 metres to the other end of the factory!
© Graham Huskinson 2010
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