GR8BIT Knowledge base article #KB0014

GR8BIT: the Real Engineering Experience
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Nov 08, 2013Severity: InformationEugeny Brychkov (RU)

Composite and Y/C Video Board

Background: In the default configuration GR8BIT has two video outputs:

It is a great pity that not every TV or monitor feature RGB input which support 15kHz dot clock frequency, and sometimes it is really hard to get full fun of color picture. Here we offer easy solution, and we hope you will get fun from building the small board which will allow you to get color picture through CVBS wire and S-video interfaces.

Solution: Our recommendation is to use single NTSC/PAL encoder chip; we selected CXA1645, which is available in both DIP (P) and SOP (M) packages. The schematic shown on fig. 1 uses encoder chip in the NTSC mode, which is default for GR8BIT (60Hz frame rate). To have PAL TV connected to the GR8BIT, you will need to put CXA1645's pin 7 to ground instead of +5v, attach 4.43MHz clock source to the R6's left pin, and put VDP into the PAL mode by setting register #9 bit *NT to 1 typing from BASIC prompt "VDP(10)=VDP(10)OR2".

4.43Mhz clock source is missing in the GR8BIT, thus it will be externally brought component (for example, quartz generator). Opportunity to choose between 50/60Hz modes would be the next improvement item for the GR8BIT language pack.

Figure 2 shows the PCB. You may not need to use these images if you decide to build circuit in point-to-point wiring manner (fig. 3). What you should remember that this additional board is installed above the GR8BIT Video board, and you should have components which you solder on this board's inner surface (which goes face-to-face to Video board) to fit empty space on the Video board.

Composite and Y/C Video Board schematic
Figure 1. Circuit diagram of the Composite and Y/C Video Board in NTSC mode

Composite and Y/C Video Board front/outer surface Composite and Y/C Video Board back/inner surface
(a) outer surface (b) inner surface
Figure 2. PCB layout

Assembly instruction: here is step-by-step point-to-point wiring method assembly instruction. While designing your and assembling, remember that components on the board should fit into the space empty from component bodies mounted on the Video board. Also, height of the whole assembly should not be higher than height of one standard ISA card unit (i.e. width of the bracket), otherwise you will not be able to install other cards to the right of video board.

Needed components
(a) collect all needed components
  Mark dimensions
(b) mark the dimensions of the board and positions of the holes with marker
Cut long edge
(c) cut long edge
  Cut next edge
(d) cut the next
Cut small edge
(e) cut another small one
  Scarify inner dimensions
(f) scarify inner dimensions
Break off the pieces
(g) and break off the pieces out
  Scarify another dimension
(h) same with another inner dimension
Break off
(i) and break off
  Drill mounting holes
(j) drill the mounting holes
Use file to smooth dimensions
(k) use file to smooth dimensions
  Drill connectors' mounting holes
(l) find out exact location of the CN1. Locations of connectors
and drill mounting holes for them
Check height
(m) check the height: CN1 should firmly fit into video board's CN4.
If not - file dimensions of the top of connectors down
  Solder connectors
(n) then solder connectors. Board at the points of arrows should
be almost adjoin (lie on) the video board's DB-9 connector's body
Solder components to outer surface
(o) solder components onto outer surface
  Solder components to inner surface
(p) solder components onto the inner surface
Wire from inner side
(q) wire from the inner side
  Wire from puter side
(r) wire from the outer side
Connect boards
(s) put boards together
  Get the color!
(t) and you will get the color
Figure 3. Process of making the board

Sources and information for further reading

End of KB0014 "Composite and Y/C Video Board".

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