IBM SG24-4576-00 manual Scsi Adapters, ∙ Synchronous versus Asynchronous

SCSI bus bandwidth. If the controller held onto the bus while waiting for the device to seek, then the other devices would be locked out. This is also sometimes referred to as overlapped operations or multi-threaded I/O on the SCSI bus. This feature is very important in multitasking environments.

∙Synchronous versus Asynchronous

An asynchronous device must acknowledge each byte as it comes from the controller. Synchronous devices may transfer data in bursts and the acknowledgments happen after the fact. The latter is much faster than the former and most newer devices support this mode of operation. The adapters negotiate with devices on the SCSI bus to ensure that the mode and data transfer rates are acceptable to both the host adapter and the devices. This process prevents data from being lost and ensures that data transmission is error free.

1.6.3SCSI Adapters

The SCSI adapter provides the interface between the host bus (for example Micro Channel or PCI) and the SCSI bus. The SCSI adapters that IBM has developed are:

∙IBM Personal System/2 Micro Channel SCSI Adapter

This adapter is a 16-bit Micro Channel bus master adapter adhering to the SCSI-I interface. It is capable of an 8.3 MBps burst data transfer rate on the Micro Channel. It uses a 16-bit data path and can use a 24- or a 32-bit address on the Micro Channel. It can be installed in either a 16- or 32-bit MCA slot, but if the system has more than 16MB of memory, it must be put in a 32-bit slot due to the limitations of 24-bit addressing in a 16-bit slot.

The bus master capability of this SCSI adapter optimizes data flow from each SCSI device configured to the system. This capability can provide performance benefits in applications where multitasking or high-speed data flow is essential. It allows the processor to be off-loaded from many of the input/output activities common to DASD transfers. This adapter also conforms to the Subsystem Control Block (SCB) architecture for Micro Channel bus masters.

∙IBM Personal System/2 Micro Channel SCSI Adapter with Cache

This adapter provides a superset of the features of the PS/2 Micro Channel SCSI Adapter. It is a 32-bit Micro Channel bus master adapter containing a 512KB cache buffer. The cache is used to buffer data between system memory and the device, which permits higher efficiency on both the Micro Channel and the SCSI buses. It has a burst data transfer rate on the Micro Channel of 16.6 MBps. This adapter is recommended where improved data transfer rates and multiple SCSI devices are required and system memory is constrained.

∙IBM SCSI-2 Fast/Wide Adapter/A

This adapter is a high performance SCSI-II adapter and a 32-bit Micro Channel bus master adapter capable of streaming data at 40 MBps. It has dual SCSI-II fast and wide channels (one 20 MBps internal and one 20 MBps external). It supports devices using either asynchronous, synchronous, or fast synchronous (10 MBps) SCSI data transfer rates. It also supports

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