Tech note October 98
In some configurations, when a MACCELERATE!750 processor boards is installed together with an Adaptec 2940UW SCSI controller, the hard drives connected to that controller will not boot the system any more, while they are recognized and operate well after the system is started from a different hard drive or CD ROM connected to the internal SCSI controller. This behaviour changes with different versions of the Adaptec controller; in a test configuration (an UMAX Pulsar with a MACCELERATE!750-300 and an Adaptec UW2940 w/ 4 GB Seagate UW drive), the system was booting from the drive connected to the Adaptec controller when Adaptec's firmware revision 2.5 was installed in the controller; after upgrading the controller to firm revision 3.0, the system refused to boot, but - after some changes to the drive settings - recognized and operated the Seagate drive when the system was started form a hard drive connected to the internal SCSI controller.
The reason for this behaviour is currently not known, also there is no known workaround. As reports said that similar problems had been recognized in other G3-based systems and configurations, and as the behaviour of the Adaptec 2940 UW controller changes with different firmware revisions, we must assume that this problem is software-related on the side of the Adaptec 2940 UW controller. Customer who experience such a problem are asked to contact Adaptec support for a solution.
At the time beeing, we can't say which kind of problems may appear when operating the MACCELERATE!750 together with an Adaptec 2940UW SCSI controller; problems such as the above described may or may not appear, and it may be possible that worse problems are encountered. We highly recommend all users who consider to use the MACCELERATE!750 together with an Adaptec 2940UW SCSI controller, to follow the backup and safety advises in the MACCELERATE!750 manual before installing such a configuration.
Similar problems of this kind have not been encountered with other PCI-based UW SCSI controllers, such as the Atto ExpressPCI, which is tested to work trouble-free with the MACCELERATE!750 processor boards.
Bus speed settings of the MACCELERATE!750 in different Apple or Clone system
The MACCELERATE!750 processor boards have a bus speed selector, which allows the user to select different bus speed settings in his system. Faster bus speed settings usually gain a small performance increase of a few percent in real world applications; it must be noted, however, that not all systems will run with bus speed settings beyond the system's specified clock. This means that some systems, which are specified for a bus speed of 50 MHz, may work well with 55MHz, but others not. There are also older PowerMacintosh models on the market which had original CPU cards with a bus speed of 40 or 44 MHz; these older systems may not even run with 50MHz bus speed settings. Due to the differences between the many systems in the market, there is unfortunately no guarantee that faster settings will operate in a particular system.
In all system configurations, it is highly recommended to remove an on-board second level cache, provided it is installed in form of a Cache DIMM (as in all PowerMacs 7300, 7500, 7600, 8500 and 8600).