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:date: 2025.12.09 04:57:07
:_modified: 1970.01.01 00:00:00
:tags: HW,MHF-002
:authors: Gilhad
:summary: Composite_output
:title: Composite_output
:nice_title: |logo| %title% |logo|

%HEADER%

Composite_output
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The composite output is currently done via USART running on half system clock.

One line is 64 uS long.

Sync pulse should be 0 V, black should be 0.3 V and white should be 1 V.

|RCA_schema.png|

Logic signal goes to resistor divider over 440 Ohm to joint of 1k pullup to +5V and the 50 Ohm screen input. This was computed to make the requested 0.3 - 1 V levels.

Sync goes transistor (over NOT gate and 1k3 resistor) to pull that to 0 V. (and the resisors of divider limits the current from gates.)

As the USART lets its pin to flow, when disabled, the USART output goes via AND gate and Suppress signal keep the output LOW when no data are transmited.

In future (in this version) I want try to feed it from 74HC166 in similar way as VGA does it, to have 2x faster data here (same as system clock)

The jumper 40-80 selects the source, either USART (marked as 40 chars per line) or the 74HC166 output (marked 80 chars)

Here is the part on PCB

|RCA_top.png|
(top)

|RCA_bottom.png|
(bottom)

The RCA uses 1/4 AND gates of 74HC008 (the rest is avaiable as 3xAND blocks for future use) and 1/6 NOT gates of 74HC14 (4/6 are used by PS/2 and HALT, 1/6 is avaiable here too)

Here is osciloscope showing the RCA signal (magenta Out), Supress (green) and clock from PS/2 (yellow). The cyan Int is some debug.

Here we see the PS/2 traffic - pulses of similar width as RCA lines, but no synchronisation possible. Magenta Out show 0V sync pulses and 0.3V-1V data accompanied dark area before and after visible line.

Here is osciloscope of 3 full screens and 8 PS/2 codes - around vertical sync is blank area bellow and above visible screen and the sync is way longer there.

There is no time to produce both RCA and VGA signals at the same time, but it is possible to switch the output on the fly. But the monitors are slow and take few seconds to adjust and start showing incomming signal, so no smart tricks probabelly possible here.


.. |RCA_bottom.png| image:: RCA_bottom.png
	:width: 250
	:align: top
	:target: RCA_bottom.png

.. |RCA_schema.png| image:: RCA_schema.png
	:width: 250
	:align: top
	:target: RCA_schema.png

.. |RCA_top.png| image:: RCA_top.png
	:width: 250
	:align: top
	:target: RCA_top.png