soohong@cs.psu.edu (Soohong Kim) (02/25/90)
Could anyone kindly answer my question? What does the following excerpt from ACB-4000 series user's manual would mean: "Jumper G-H is used to select two DMA transfer rates on the SCSI bus. When removed, the controller will transfer data at the maximum rate allowed by the controller, equal to DATACLOCK/2. Some host adapters cannot support this data rate. When this jumper is installed, the data transfer rate is reduced to run at a rate of SYSCLOCK/4 on single sector transfers. This is one-half of the controller's maximum DMA speed. Multisector transfers are always made at the maximum rate of DATACLOCK/2, with or without this jumper." In fact, I am using Adaptec 4070 (ST-506 to SCSI converter) for my *IBM-incompatible* PC. The host adaptor with which 4070 is connected is a home-brewed one and it uses DMA. As long as I know, since all transfer over the SCSI bus is based on asynchronous REQ/ACK handshaking, the term "clock" does not apply here, right? (I am sure that I am not using synchronous SCSI and there is nothing in the manual saying synchronous data transfer over the SCSI bus is supported by this controller.) But, indeed, as I experiment with my system, it would not transfer data correctly (I didn't try to see if overrun occurs) if I remove the jumper to select the maximum clock rate. I am very curious why this happens. But being a poor student I have no good equipments to unveil the mistery :-(. Any opinion? Please reply via e-mail. Thank you in advance. ---Soohong (soohong@psuvax1.cs.psu.edu) -- =================== soohong@psuvax1.cs.psu.edu soohong@psuvax1.bitnet UUCP: psuvax1!soohong USPS: 333 Whitmore lab, University Park, PA 16802 phone:(814)234-0938