Use the Spectrum 128 style menu below to navigate to the various sections detailing the SCART cable.

Lead Circuitry

The Spectrum 128 can be connected directly to a SCART socket via composite video (CVBS) as shown in the circuit diagram below. A 15 ohm resistor could be placed in line with the CVBS signal to reduce the 1.2V signal out from the Spectrum 128 down to the 1V level required by the SCART socket but this is really not necessary due to the input tolerance accepted by the SCART socket. For best results, use screened cable and attach this to the connector shields at both end.

Connecting the Spectrum 128 to a SCART socket via RGB is a little more involved, requiring the three separate colour signals and a synchronisation signal. The CVBS and GND signals should again be connected as shown above. Even though an RGB connection only requires synchronisation information to be delivered to SCART socket's VIDEO IN line, the CVBS signal can be used as the TV set will generally ignore the picture information and just extract the synchronisation information required. The diagram below shows the circuit required for each of the colour signals.

The 150 ohm resistor reduces the colour signal down to the maximum level suitable for the SCART socket. When the BRIGHT signal is high, it has no effect on the colour signal level due to the diode and hence the bright shade of the colour is produced. For the normal intensity colour, the BRIGHT signal is low and the diode conducts thereby pulling down the signal level coming out of the Spectrum 128.

To specify the TV to automatically display an RGB input when the SCART socket is selected requires a voltage of 1V to 3V to be applied to the SCART socket's BLANKING input (pin 16). A 1V signal can be obtained by directly connecting the CSYNC signal from the Spectrum 128 to the BLANKING input, as shown below.

If the TV set allows manual selection of the AV mode that the SCART socket displays, i.e. RGB or composite video, then the CSYNC to BLANKING connection can be omitted, and this has the benefit that the same lead can then be used to view either an RGB or composite video picture. If the BLANKING input is left disconnected then the composite VIDEO INput will be displayed by default.

A drawback of the Spectrum 128 monitor socket is that, unlike the Spectrum +2A and +3, it does not provide a sound output. It is therefore necessary to obtain the sound signal from the MIC cassette port via a 3.5mm Jack plug. The SCART socket has separate connections for left and right audio channels and so these must be wired together. The voltage from the Spectrum 128 MIC socket is typically 200mV and this is directly suitable for driving the SCART socket's audio inputs. The SCART socket's audio ground can either be connected to the video grounds inside the SCART plug or to the ground signal from the MIC socket, but should not be connected to both as this might create a ground loop. The wiring of the audio signals is shown below.

Putting all of the above together, the complete wiring of a RGB SCART lead for the Spectrum 128 is shown below. For best results, use screened cable and attach this to the connector shields at both end.

In the very rare cases that problems are experienced when using the composite video signal to drive the SCART socket's VIDEO IN line then the composite sync signal from the Spectrum 128 should be connected instead, as shown in the circuit below. The BLANKING input (if required) must then be driven by an alternative voltage source.

When CVBS can be used to the drive the VIDEO IN line, the CSYNC signal from the Spectrum 128 can be connected to the BLANKING input and will apply exactly 1V. However, some TV sets may be very harsh and deem this to be below the level needed to select the RGB input. If this is the case then a higher voltage must be obtained from elsewhere, and unfortunately the Spectrum 128 monitor socket does not provide a power output line. However, power can be obtained from either the KEYPAD socket, RS232 socket or the edge connector. Alternatively, an external 3V power supply can be used to directly drive the BLANKING input, in which case the power supply's ground output should be connected to the BLANKING GND input (pin 18) of the SCART socket. If the BLANKING input is connected via any of the above methods then the BLANKING GND should also be connected to 0V at the Spectrum 128.

The KEYPAD or RS232 socket's +12V output can be used to apply 2.5V to the SCART socket's BLANKING input using the circuit shown below.

The +5V output on the edge connector or an external power supply (up to +12V) can be used to drive the BLANKING input as shown below. This uses a voltage regulator to generate a 1.67V level for the BLANKING input.

In the unlikely event that manual selection of the SCART socket is not available then +9.5V to +12V must be applied to the FUNCTION SWITCHING input (pin 8) of the SCART socket. Power can be obtained directly from the KEYPAD socket, the RS232 socket, the edge connector, or an external power supply. The KEYPAD and RS232 socket's +12V output can be used to directly drive the FUNCTION SWITCHING input directly and the BLANKING input via a 61 ohms resistance. This results in 9.5V on the on FUNCTION SWITCHING input and 1V on the BLANKING input. These are just within the required ranges and so it would be better to drive the BLANKING input from the CSYNC signal from the Spectrum 128 and drive the FUNCTION SWITCHING signal from the +12V output from the KEYPAD or RS232 socket. If the FUNCTION SWITCHING input is connected via any means then the BLANKING GND should also be connected to 0V at the Spectrum 128.

It is also possible to drive the BLANKING line using a combination of CSYNC and a voltage regulator, as shown below. If there is no external power supply connected then the only voltage applied to the BLANKING input will come from CSYNC. If an external power supply is connected then the output from the voltage regulator of 1.67V is also connected back to CSYNC, but fortunately this is suffciently isolated due to a 68 ohm resistor within the Spectrum 128.

The +12V output from the edge connector or from an external +12V power supply can be used to drive the FUNCTION SWITCHING and BLANKING inputs as shown below. A voltage regulator is used to generate a 1.67V level for the BLANKING input. The CSYNC signal can also be connected to the BLANKING line if desired.

Full circuit calculations can be found be here.