Current gerbers are here.
Current Kicad files are here.
Errata : The output of U6b pin 6 must be inverted before pin 1 of U10. i.e. the direction signal is incorrect. Updated in version 001B
SW1 sets the base port address for the DMA board. Two ports are decoded for the DMA board from anywhere within the Z80 port range of 00-FFh.
|Base + 0||E0h||DMA Chip access|
|Base + 1||E1h||RDY control signal|
Note that A0 is not set on SW1 and A7-A1 are in reverse order. A binary 0 is set by a jumper. A binary 1 is set by having no jumper.
Example for setting SW1 to suit base address E0h
E0H (A7-A0) = 11100000b
Reversed (A0-A7) = 00000111b
Drop A0, (A1-A7) = 0000111b
Set all 0 positions with a (J)umper. All 1 positions are left (U)njumpered
Theory of Operation
Interrupt Logic Support.
The Z80 DMA supports interrupts. 74LS08 U1A and U1B implement the Z80 interrupt daisy chain with lookahead logic.
Board Select Logic
74LS688 provides port selection logic for 2 I/O ports and is qualified by ~M1 and ~IORQ.
The ~CE_DMA_BRD signal is generated and is used to qualify the Z80 DMA ~CE/~WAIT line and the data direction logic.
Data Direction Logic
74LS245 U10 bus transceiver controls the direction of the data bus. When the DMA board is idle the direction is “data-in”.
The data direction will switch to “data-out” if:
- a read is made to the base port address