The AHA-1740A/1742A/1744 supports asynchronous, synchro- nous, and Fast SCSI data transfers on the SCSI bus. Asynchro- nous data transfers support up to 2 MBytes/sec, synchronous transfers support up to 5 MBytes/sec, and Fast synchronous trans- fers support up to 10 MBytes/sec. Devices using different trans- fer rates can all be used on the same SCSI cable. The AHA-1740A/1742A/1744 is both SCSI-1 and SCSI-2 compatible, and supports both SCSI-1 and SCSI-2 devices.
The AHA-1740A/1742A/1744 fully supports the SCSI features of disconnect/reconnect, zero latency reads, and tagged queuing. Dis- connect/reconnect support maximizes SCSI bus utilization for multiple target systems. Zero latency reads and tagged queuing minimizes the mechanical limitations of SCSI fixed disk drives.
The AHA-1740A/1742A/1744 has an on board BIOS that allows it to be used in addition to, or in place of, a standard fixed disk con- troller. Thus, up to two SCSI fixed disk drives can be used under DOS without the use of device drivers. Up to seven SCSI fixed disk drives can be used without a driver if MS-DOS 5.0 or higher is used. Booting is also supported by the BIOS in the DOS envi- ronment.
The AHA-1740A/1742A/1744 fully supports scatter/gather, a fea- ture utilized by device drivers to minimizes the number of I/O commands into memory. The UNIX®/XENIX® and NetWare® op- erating environments take advantage of this feature.
The AHA-1740A/1742A/1744 has two modes of operation: stand- ard mode and enhanced mode. The standard mode is software compatible with the AHA-1540 series host adapter drivers. The enhanced mode uses an advanced, high performance mailbox interface and features full EISA 32-bit addressing. The AHA-1740A/1742A/1744 in enhanced mode can support up to
4 Gigabytes of host RAM memory, while standard mode is limited to 16 MBytes of host RAM. The operation mode of the AHA-1740A/1742A/1744 is selected by running the EISA Configu- ration utility program provided with every EISA machine. The modes of operation are fully described in Chapter 6, Host Adapter BIOS Operation.
