8- And 16-Bit Memory And Odd Byte Data Transfers; Bus Auxiliary Interface Chip (Aic-565) - Adaptec AHA-1740A Technical Reference Manual

EISA-to-Fast SCSI Host Adapter
synchronous and asynchronous peripherals can be freely mixed on a cable connected
to the AHA-1740A/1742A/1744.
In addition, the host adapter has the ability to select particular targets during con-
figuration to initiate synchronous negotiation, enable parity checking, send start-up,
and allow disconnection. The current limiting fuse on other host adapters has been re-
placed with a thermistor to allow nondestructive current limiting of terminator
power supplied to the SCSI cable.
The AHA-1740A/1742A/1744 utilizes the Adaptec AIC-6251 Fast SCSI protocol chip
to maximize the SCSI bus utilization. The AIC-6251 is an Adaptec VLSI development
of the popular AIC-6250 device which allows the AHA-1740A/1742A/1744 to achieve
greater than 2.0 MBytes/second asynchronous SCSI data transfer rates, and up to 10
MBytes/second synchronous data transfer rates. The AIC-6251 will also enable the
AHA-1740A/1742A/1744 to operate simultaneously as both an initiator and as a proc-
essor target device.
Through a 16-bit interface internal to the board, the AIC-6251 reduces bus busy time
during data transfer by allowing a bursting data across the EISA bus at up to 33
MBytes/second. The AIC-6251 has separate data buses for the local microprocessor
and for the system data bus. This further increases the performance of the AHA-
1740A/1742A/1744 by reducing the overhead associated with SCSI commands.

8- and 16-Bit Memory and Odd Byte Data Transfers

The AHA-1740A/1742A/1744 will automatically shift to 8- or 16-bit data transfers as
indicated by the control lines on the EISA bus. Bus master data transfers into 8- or
16-bit wide memory are fully supported as are full 32-bit wide data transfers.
During normal DMA operations, nearly all transfers to and from memory are 32-bit
transfers. At the very end, or the very beginning of an odd address boundary, an 8-
bit, 16-bit, or 24-bit transfer may occur.

Bus Auxiliary Interface Chip (AIC-565)

This highly integrated ASIC (Application Specific Integrated Circuit) device devel-
oped by Adaptec is also used with the AHA-1540B host adapter family. It provides
the Standard Mode programming interface and implements the following main func-
tional blocks:
Bus control interface for Standard Mode
Host adapter microprocessor interface
BIOS decode logic
The bus control interface section decodes the possible base addresses for the board
I/O port address, including a select external to the board. It also provides all registers
used to communicate between the host adapter and the motherboard which are ac-
cessed through the bus in Standard Mode. This ensures full compatibility with soft-
ware written for the AHA-1540/1542 family of AT Bus Master Host Adapters.
Architecture
2-3