Commodore Amiga A500 Technical Reference Manual page 11

Full Bus Termination
Internal RAM
Expansion on the A500
EIA Ring Indicate
Support
External Disk Connector Defined Identifications
$0000 0000 - no drive present
$FFFF FFFF - Amiga standard 3.25 diskette
$5555 5555 - 48 TPI double density double sided
As with other peripheral ID's, users should con-
tact Commodore Technical Support for ID
Assignment.
The serial input data is active low and must there-
fore be inverted to be consistent with the above
table.
External Disk Connector Limitations
1. The total cable length including daisy chaining
2. A maximum of 3 external devices may reside
3. Each device must provide a 1000 Ohm pullup
Unlike the A1000 and the A500, both versions of the Amiga 2000
have an internal expansion bus, as a function of having an internal
card cage.
On the A500, memory at $C00000 is "slow" RAM (the processor is
locked out by the custom chips) rather than fast RAM as suggested
by A1000 external expansion. Thus, when ExecBase is transferred
to $C00000 to free up chip RAM, there is no speed advantage.
However, you would still be making real chip RAM available for
other purposes. The B2000 functions as the A500 does in this
regard.
The A500. A2000 and B2000 support the RS232 RI lead to allow
operation with modem standards. When the RI signal is asserted,
the parallel port SEL line will be driven low. If this function is not
desired, the RI lead should be disconnected in the modem cable.
8
must not exceed 1 meter.
on this interface (2 for the A2000).
resistor on every open collector input.