Up to this point, we have only focused on the basic architecture of the system and went into detail on the graphics card, and CPU card, only briefly mentioning the data and I/O cards. 

But before we can begin to get a hello world program on the system, we must take a closer look at the I/O card. 

In a previous blog post, we mentioned the schematic of the I/O card, however to keep things simple, we will post the schematic again here: 

With that, here is the system architecture of said card: 

Peripheral - I/O Card

8051 Memory Map:
$0000 - $01FF = RAM
$0200 	      = UART Data
$0201         = UART Command
$8000         = PIO1 Command/Status Register
$8001         = PIO1 PORTA (VME Address/Data AD1-AD8)
$8002         = PIO1 PORTB (Rear External Switches 1-8)
$8003         = PIO1 PORTC (Various)
$8004         = PIO1 TCNT0 Low
$8005         = PIO1 TCNT0 High
$8100         = PIO2 Command/Status Register
$8101         = PIO2 PORTA (Indicator LEDs)
$8102         = PIO2 PORTB (DTMF Dialer Codes, 1 Indicator LED)
$8103         = PIO2 PORTC (Bus Clear, Charging Indicator)
$8104         = PIO2 TCNT0 Low
$8105         = PIO2 TCNT0 High

I/O Card VME Addressing:
$C00000 to $C001FF = 8051 INT0
$C04000 to $C041FF = 8051 INT1

PIO-1:
PA0: VME Address/Data 0
PA1: VME Address/Data 1
PA2: VME Address/Data 2
PA3: VME Address/Data 3
PA4: VME Address/Data 4
PA5: VME Address/Data 5
PA6: VME Address/Data 6
PA7: VME Address/Data 7

PB0: Switch 1
PB1: Switch 2
PB2: Switch 3
PB3: Switch 4
PB4: Switch 5
PB5: Switch 6
PB6: Switch 7
PB7: Switch 8

PC0: (LED 7) Modem In Use LED
PC1: Modem ALB
PC2: Modem SQT
PC3: Modem/AT Serial Select
PC4: Cube Relay Control
PC5: VME IRQ2

PIO-2:
PA0: (LED 10) System Error LED
PA1: (LED 8) Spare LED
PA2: (LED 2) Sat Data LED
PA3: (LED 3) Local Video LED
PA4: (LED 6) On Air LED
PA5: (LED 4) Spare LED
PA6: (LED 1) Satellite Video LED
PA7: (LED 5) Pre-Roll LED

PB0: DTMF Keypad Column 1 
PB1: DTMF Keypad Column 2
PB2: DTMF Keypad Column 3
PB3: DTMF Keypad Row 1
PB4: DTMF Keypad Row 2
PB5: DTMF Keypad Row 3
PB6: DTMF Keypad Row 4
PB7: (LED 9) Spare LED

PC0: VME IACK2
PC1: Battery Charging Indicator
PC2 - PC5 unused. 

This is basically where everything is hooked up as far as the address map. Essentially, it is an 8051 based card, with two PIO/RAM ICs, and a UART IC. 

The card handles the LEDs, a modem, AT keyboard, serial port, and some general purpose I/O. 

The funny thing about this card however, is the 68K bus interface logic and timing. It is all software-based and its controlled via interrupt on the 8051, so the interrupt execution time has to be in-step with the 68K Bus Cycles. absolutely insane, and there is NO room for timing variances or error on the bus read/write interrupt handlers or errors will ensue. I tried it. 

One thing i noticed right away on the design of this particular card, is how there is an inverter off of the Q output on the flipflop, instead of just using the /Q output. As seen here:

This part made no sense to me. You would save a gate if you just wired the /Q output instead of using a makeshift inverter. Maybe there is a timing reason? Honestly, I do not know. 

In the next part, we need to disassemble the ROM and take a look at what is going on, since we have no driver or any details on how to drive this card.