Commodore Computers 1581 - RAM Usage Map; Serial Interface Communication

1581 SERVICE MANUAL
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6 MEMORY MAP — RAM USAGE
$0002-$00FF Zero Page Variables
$0100-$01FF Vectors, Variables and Stack Area
$0200-$02FF Command/Error Buffer and Variables
$0300-$03FF Buffer #0
$0400-$04FF Buffer #1
$0500-$05FF Buffer #2
$0600-$06FF Buffer #3
$0700-$07FF Buffer #4
$0800-$08FF Buffer #5
$0900-$09FF Buffer #6
$0A00-$0AFF BAM
$0B00-$0BFF BAM
$0C00-$1FFF Track Cache Buffer
7 COMMODORE SERIAL INTERFACE
The 1581 supports Standard, Fast, and Fast Serial communication like the 1571.
A HRF (Host Request Fast) command places the drive in fast serial mode. The 1581 remains in
fast serial mode until an Unlisten, Untalk, or serial bus error. The 1581 will also source a DRF (Device
Request Fast) message. This message lets the host know that the addressed peripheral can receive
bytes fast (or slow).
The bus consists of the following:
PIN 1 — SRQ (Service Request)
Unused by the current serial bus. Fast serial will use this line as a bi-direction fast clock line.
PIN 2 — GND
Chassis ground.
PIN 3 — ATN(in)
The host brings this signal low which then generates an interrupt on the controller board. The
attention sequence is followed by an address. If the device does not respond within a preset
time, the host will assume the device addressed is not on the bus.
PIN 4 — CLK (in/out)
This signal is used for timing the data sent on the serial bus (software clocked).
PIN 5 — DATA (in/out)
Data on the serial bus is transmitted one bit at a time (software toggled). In addition, this line
is wire 'ored' and used as a FAST DATA line to compliment the FAST CLOCK on the SRQ line.
PIN 6 — RESET
> This line will reset the peripheral upon host reset.