SVI-3x8 PPI
This page was last modified 18:20, 11 April 2021 by NYYRIKKI. Based on work by Gdx.

SVI uses same 8255 PPI as MSX. Both machines use "MODE 0" communication, but it has different function in the system.

Here is how it is connected:

I/O port R/W Description
94 W PPI 8255 Port A
95 W PPI 8255 Port B (Can't be used)
96 W PPI 8255 Port C
97 W PPI 8255 Mode select and I/O setup of A,B,C
98 R PPI 8255 Port A
99 R PPI 8255 Port B
9A R PPI 8255 Port C
9B R PPI 8255 Mode select and I/O setup of A,B,C

Contents

Mode select I/O Port address #97 (out) / #9B (in)

The ports A and B are input and port C is output. This means that in any normal situation the mode select byte should be #92 (See 8255 manual) Exception to this rule is how ever, if computer needs to communicate outside trough joystick ports (ie. while reading graphics pad). In this case the port A can be changed to output (Mode select byte = #82) Please note: Writing mode byte will reset the ports and therefore turn cassette motor on.

This port can be used also to toggle bits in port C. (See 8255 manual)

Port A I/O Port address: #98 (In) / #94 (Out)

  • Bit 7: Cassette: Read data
  • Bit 6: Cassette: Ready status
  • Bit 5: Joystick 2: Trigger (pin 6)
  • Bit 4: Joystick 1: Trigger (pin 6)
  • Bit 3: Joystick 2: EOC (pin 9)
  • Bit 2: Joystick 2: /SENSE (pin 5)
  • Bit 1: Joystick 1: EOC (pin 9)
  • Bit 0: Joystick 1: /SENSE (pin 5)

Please see: SVI-3x8 Joystick port

Port B Input Port address: #99

  • Bit 7: Keyboard: Column status of selected line
  • Bit 6: Keyboard: Column status of selected line
  • Bit 5: Keyboard: Column status of selected line
  • Bit 4: Keyboard: Column status of selected line
  • Bit 3: Keyboard: Column status of selected line
  • Bit 2: Keyboard: Column status of selected line
  • Bit 1: Keyboard: Column status of selected line
  • Bit 0: Keyboard: Column status of selected line

Please see: SVI-3x8 keyboard

Port C output Port address: #96

  • Bit 7: Key click sound output (pulse)
  • Bit 6: Cassette: Audio output (1=Audio output enabled, only available in SV-903, not in SV-904)
  • Bit 5: Cassette: Write data
  • Bit 4: Cassette: Motor relay control (0=on, 1=off)
  • Bit 3: Keyboard: Line select 3
  • Bit 2: Keyboard: Line select 2
  • Bit 1: Keyboard: Line select 1
  • Bit 0: Keyboard: Line select 0