5.5.2 1770 FDC The 1770 is expected to be the standard disc interface. A 1770 operates in either single density (FM) or double density (MFM) mode, and has data separator and disc speed decision logic built in. A much simpler disc interface results: ICs 1, 2, 3, 4 and 9 are not needed. No drive select logic is incorporated in a 1770 so IC17 is fitted for this function. IC17 also latches two contol signals which are used to select between FM and MFM operation and to reset the 1770, both under program control. The control register IC17 is a write only device which occupies the address space &FE80 to &FE83.
IC23 gates the decoded address signal with notW (IC27 pin 6) to form the control register clock. All 1770 registers are addressed in the range &FE84 to &FE87. It can be seen that the 1770 controller and the 8271 controller address space has been swapped. This is to allow the disc system software to distinguish between the two devices. Two interrupt signals come from a 1770, pins 27 and 28. The two interrupts are inverted and wire NORed on to the notNMI line by two parts of IC7 (quad NAND gate). Link S8 selects between the single interrupt of an 8271 and dual interrupt of a 1770. When a 1770 is fitted S8 must be made.
For the 1770 option link S7 is made East, this incorporates not2M into the chip select signals and so defines the timing of data transfers between the disc controller and the CPU. Link S5 is available to allow program controlled suppression of 1770 disc controller interrupts (IC17 is not present when the 8271 is fitted). The disable function is not used at present so S5 is not fitted. 5.6 Display circuitry Three display outputs are provided - RGB, composite video, and UHF. 5.6.1 RGB Red, green and blue signals are produced by the video processor and are then buffered by Ql, Q2, and Q3 to be fed to the DIN socket (SK3) at TTL type levels.
The fourth signal required at the RGB output is a composite SYNC (CSYNC) generated from horizontal sync and vertical sync of the 6845. CSYNC polarity can be altered using link S27. The 0V and +5V power supply also appears on SK3. 5.6.2 Composite video Composite video is a summation of the three primaries (red green and blue) to give a grey scale, mixed with negative CSYNC. The order from darkest to lightest is: black, blue, red, green, magenta, cyan, yellow, white. The grey scale is set by the resistor values 8101, 8104, and 8108. These feed Q8 which produces a 1V peak to peak signal on BNC connector SK2.
The chroma component described in 5.6.3 may be added to the composite video by making link S26. The voltage ratio of chroma to luminance is defined by C48. 21