CAT5 VGA Extender TRVGA300

View of the transmitter

The front panel of the receiver

The rear panel of the receiver, with DIP switches

Distinguishing features:

The TRVGA300 transmitter/receiver set is used for active transmission of VGA signals via a UTP cable, over distances up to 300 m. They are complementary devices - the transmitter changes the VGA signals into a balanced form suitable for transmission via twisted pairs of CAT5 cable, the receiver converts them back into the original form. Three pairs are used for transmission of RGB components, while the fourth for sending synchronization signal.

On the back of the receiver there are switches for compensation of UTP/FTP cable characteristics. Each component color and the synchronization signal has an individual switch. All switches must be in the same position for a given distance. The real resolution of the output signal depends on the mode of the input signal and the distance of transmission. The DIP switches allow to correct transmission parameters for distances from 0 to 300 meters with a step of 50 m. The correct settings for the corresponding lengths are presented in the following figure.

Settings of the DIP switches for different lengths of a UTP/FTP cable

Due to the fact that each color is sent via a separate twisted pair of the UTP/FTP cable and the pairs do not have identical lengths, the colors at the output may be slightly shifted in comparison with the original signal. It is more visible in the case of larger distances - up to 2 pixels for 300 m. This phenomenon can be practically eliminated by equalizing the lengths of the pairs and resultant parameters of each path by "interlacing" them - dividing the distance between the transmitter and receiver in three equal parts and changing the pairs after each part so that each component color is transmitted via three different pairs. This idea is presented in the following figure. The synchronization signal is transmitted over the same pair.

The idea of "interlacing" the pairs in a UTP/FTP cable in order to compensate the differences in lengths and parameters

Another way of compensating the shift of the component colors, especially when the division of the cable may be difficult or impossible, is to increase the lengths of the shorter pairs. The easiest way to check the differences in lengths is by watching the white vertical line with a width of one pixel on a black background on the monitor screen (fig. a). It is easy to generate such a line using e.g. the "Paint" program. The differences in length appear on the screen as color sub-lines.

With a resolution of 1280x1024@60Hz, the duration of the horizontal line is 14.6 us. This gives 11.4 ns (14.6 us/1280) for the point (pixel). Electrical signal moves in the twisted pair at approximately 192 million m/s. The equivalent of one-pixel shift can be calculated from the formula: 192 000 0000 000 000 * 0.000 000 0114 = 2.19 m. If, for example, some component color is shifted by 2 pixels (from the most-right color - see the fig. b) it means that the corresponding pair should be extended by 4.4m. By adjusting the extra length it is possible to eliminate this "dispersion". The phenomenon is negligible for distances up to 50 m.

To measure the differences in length, one can also use an ohmmeter - the shorter pairs will have lower resistance

The phenomenon of split colors after passing through a long UTP/FTP cable

Pinout description