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
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