Monday, April 30, 2012

RS232 or "standard" serial cables


Why does my RSS232 cable not work or the myth of the RS 232 not working debunked.

RS 232 serial is an ANSI standard, but there are so many variations of this standard implemented, that it is, for all intents and purposes, not a standard any more…
RS 232 is applicable to both DB9 and DB25 connectors. The connector on a PC is always a male connector; therefore, the connector on the cable must be a female connector. You can see from the tables above, that there are transfer and receive signals, so-called hand shake signals and a chassis- and a reference ground.
Whether the receive and transmit lines are connected cross over or straight depends on what kind of device (peripheral) one wants to link to a PC. The device can either be DTE, or DCE. A PC is normally a DTE device (data terminal equipment) a modem would be a DCE device (data communication device). The problem is that you normally don’t know what kind of device your peripheral is.
Another problem is the length of the cable, the standard allows for a maximum length of 15 m, but we have successfully implemented cables with 200 m and more, the links depends completely on your computer and your peripheral.
Perhaps we should here give a short explanation of what a serial standard is. The most common standards are RS232, USB, FireWire and SATA.
In these standards data is transmitted in a serial method, that means that all characters which are sent one after the other on one wire and all received characters are on a different wire, in comparison to a parallel method where all the bits are transmitted on a different wire.
Although all the above are serial standards the way of transmitting is completely different, therefore, a ‘translation’ has to be made from one standard to the other.
From the above, it might become apparent that a conversion from USB to RS232 can be difficult if one does not know what kind of device the RS232 peripheral is.
But if one knows, what type of device is to be connected, a cable can be built to change from the standard DTE to DCE and the required handshake.