English Amiga Board - View Single Post - Different Versions of HDToolbox or is my SCSI HD maybe failing

patrik · 05 January 2018, 02:54

According to the A590 schematics, the passive SCSI termination consists of RN1, RN2 and RN3.

It is a very bad and confusing practise to solder the termination like that to a controller with both internal and external connectors.

The issue being that you cannot correctly terminate the SCSI chain if both internal and external devices are connected: The SCSI chain will in that case have one end at the internal cable and one and the other end at the end of the external cable and you shall only terminate at ends, but that won't be possible as the controller has its soldered termination in the middle of the chain.

There is however a way where this can be correct: The SCSI specification allows a short stub cable per device from the chain, somewhere along 10cm (they won't cause reflections at that length). So if you see the controller and its termination as one end of SCSI chain and the internal cable in the A590 as a stub and don't terminate the drive, it will be correct according to specification.

However, as soon as you use a longer cable or more than one device on the internal connector in combination with external devices, this will not be correct and you must remove the soldered termination from the controller and terminate at the end of the internal cable for correct termination.

So this shouldn't really be a problem on the A590, as you normally only fit one drive with a shortish cable on the internal connector, but for example on an A2091 and all GVP cards, they should definately not have soldered the termination resistors to the controller. They should instead have had them socketed or better not any at all and instead bundled the card with an external DB25 terminator for the external connector, so you didn't have to open the computer and remove the terminator resistors when attaching external devices.

05 January 2018, 02:54	#9
patrik Registered User Join Date: Jan 2005 Location: Umeå Age: 43 Posts: 924	According to the A590 schematics, the passive SCSI termination consists of RN1, RN2 and RN3. It is a very bad and confusing practise to solder the termination like that to a controller with both internal and external connectors. The issue being that you cannot correctly terminate the SCSI chain if both internal and external devices are connected: The SCSI chain will in that case have one end at the internal cable and one and the other end at the end of the external cable and you shall only terminate at ends, but that won't be possible as the controller has its soldered termination in the middle of the chain. There is however a way where this can be correct: The SCSI specification allows a short stub cable per device from the chain, somewhere along 10cm (they won't cause reflections at that length). So if you see the controller and its termination as one end of SCSI chain and the internal cable in the A590 as a stub and don't terminate the drive, it will be correct according to specification. However, as soon as you use a longer cable or more than one device on the internal connector in combination with external devices, this will not be correct and you must remove the soldered termination from the controller and terminate at the end of the internal cable for correct termination. So this shouldn't really be a problem on the A590, as you normally only fit one drive with a shortish cable on the internal connector, but for example on an A2091 and all GVP cards, they should definately not have soldered the termination resistors to the controller. They should instead have had them socketed or better not any at all and instead bundled the card with an external DB25 terminator for the external connector, so you didn't have to open the computer and remove the terminator resistors when attaching external devices.