The VIA (Versatile Interface Adapter) is a peripheral IC which is used in the 1541 for controlling the serial bus and the stepper motor, among other things.
Technical Data
![[Picture of 6522]](pics/6522.gif)
| Pin(s) | Signal | Dir | Description |
|---|---|---|---|
| 1 | GND | - | Ground (0V). |
| 2-9 | PA0-PA7 | progr. | Port A 0-7. Port A of the VIA #1 in the 1541 is not used so that you can connect a parallel cable here for speeding up floppy operations. Some modifications are needed for the 1541c and 1541-II, though. |
| 10-17 | PB0-PB7 | progr. | Port B 0-7. |
| 18 | CB 1 | in/out | Control port B 1. When configured as input, it can be used to trigger interrupts. |
| 19 | CB 2 | in/out | Control port B 2. When configured as input, it can be used to trigger interrupts. |
| 20 | Vcc | - | Supply voltage (+5V DC). |
| 21 | /IRQ | out | Interrupt ReQuest. In the 1541, this line is connected to the /IRQ input of the 6502. |
| 22 | R/-W | in | Read/-Write. 0=read on data bus, 1=write on data bus. |
| 23 | /CS 2 | in | Chip Select 2. CS2=0 AND CS1=1 = coupled to data bus, 1=tri-state. |
| 24 | CS 1 | in | Chip Select 1. In the 1541, this line is directly connected to +5V DC. |
| 25 | ø 2* | in | Phi 2. System clock signal. All data bus action takes place only when ø2=1. |
| 26-33 | DB7-DB0 | in/out | Data bus. |
| 34 | /RES | in | RESet. 0=reset the VIA to initial state. |
| 35-38 | RS3-RS0 | in | Register Select. These four pins select on of the VIA's internal registers. In the 1541, they are connected to A3-A0 of the address bus. |
| 39 | CA 2 | in/out | Control port A 2. |
| 40 | CA 1 | in/out | Control port A 1. |
VIA #1
Pin | Dir | Function
----+-----+------------------------------
PB0 | in | DATA input from serial bus
PB1 | out | DATA output to serial bus
PB2 | in | CLOCK input from serial bus
PB3 | out | CLOCK output to serial bus
PB4 | out | 0 = ATN will be acknowledged automatically
| | 1 = ATN produces no auto-ACK
PB5 | in | DIP switch 1 (left) or jumper
PB6 | in | DIP switch 2 (right) or jumper
PB7 | in | ATN signal from serial bus
PA0 | in | State of track-0 light barrier:
| | 0 = head is over track 0
| | 1 = head is not over track 0
PA1 | out | 1570/71 bus data direction:
| | 0 = input
| | 1 = output
PA2 | out | head active (1571 only)
PA5 | out | Floppy mode and CPU frequency:
| | 0 = 1541 mode with 1 Mhz clock
| | 1 = 1571 mode with 2 Mhz clock
PA7 | in | BYTE-READY signal
VIA #2
Pin | Dir | Function
----+------+---------------------------------------------
PB0 | out | STP1 - 2nd bit of stepper control
PB1 | out | STP0 - 1st bit of stepper control
PB2 | out | 0 = motor off, 1 = motor on
PB3 | out | 0 = LED off, 1 = LED on
PB4 | in | Write protection:
| | 0 = on, 1 = off
PB5 | out | DS0 (see DS1)
PB6 | out | DS1. Density Select. DS0 and DS1 control
| | the recording rate on the disk.
PB7 | in | 1 = sync mark occurred.
PA |in/out| data for read/write electronics