Why termination power is only possible in the desktop variant?

[mikrosk]

Hi,

I have been wondering, why BlueSCSI can't use USB's power to feed back the termination power pin when using the external DB25 version?

[androda]

Hello,

The DB25 model of BlueSCSI wasn't designed with a back-feed system. There's nothing technically stopping that from being implemented, it just didn't come up as a need for the DB25.

Desktop 50 pin and the latest Centronics 50 pin can both back-power termpower from the USB port.

[mikrosk]

I see. Well, if you will be sometime thinking about a new revision, this feature would be definitely appreciated by Atari crowd -- we've got the DB25 external SCSI on the Atari TT and as far as I understand, its termination resistors need powering from the external source.

[androda]

That's good to know. We added termination feedback on the C50 for a similar reason, to power adapter cards.

[androda]

A few questions to help check my understanding:
Does the Atari TT have internal termination resistors on the motherboard? Generally those will be on an internal hard drive. And in many cases the internal hard drive can power its own termination resistors.

BlueSCSI DB25 powers its built-in termination resistors from USB or Termination Power. And because the Atari TT does not provide termination power, you have to use a USB power supply to run the BlueSCSI. So the external bus will be terminated when BlueSCSI DB25 is plugged in and powered via USB.

I'm trying to understand why termination power back-feed is needed in this case when BlueSCSI DB25 will be terminating the SCSI bus due to being powered via USB.

[mikrosk]

The TT has the following scheme:

<internal SCSI 50-pin connector> --> resistor array for termination -> <external SCSI DB25 connector>

This post: https://www.atari-forum.com/viewtopic.php?p=460456#p460456 explains it very well. So if I want to use both internal and external SCSI, there's no issue because the resistor array shall be removed.

However just the next post: https://www.atari-forum.com/viewtopic.php?p=460458#p460458 (and https://www.atari-forum.com/viewtopic.php?p=460493#p460493) explains why the termination power is needed for external devices (if there are no internal ones): because there is no termination power coming from the internal bus, it must be coming from the external one.

[androda]

because there is no termination power coming from the internal bus, it must be coming from the external one.

BlueSCSI DB25 uses USB power for termination and to run the SCSI emulation. So when it's connected externally and plugged into USB, it is terminating the SCSI bus.

[mikrosk]

True but we are talking about the other end -- termination in the Atari computer. That is the one which needs to be powered.

[androda]

True but we are talking about the other end -- termination in the Atari computer. That is the one which needs to be powered.

Are we? I didn't think we were talking about the internal side.

I don't think that providing termpower to the external DB25 port would make a difference for the internal side. Termpower for the internal bus isn't connected to the DB25 port at all, right? If external isn't connected to internal, then powering externally won't make a difference.

[mikrosk]

Actually, you may be right. I was referring to this post:

In order for terminators to work on the external SCSI port, an external SCSI device must feed term power onto the SCSI bus. This must be because the Atari TT does not put termpower on the SCSI bus at the external SCSI port.

I read it as "if you want to power the [internal] terminators, you must power it from the external device" however perhaps what is meant here is "if you want to power the external terminators and your external device doesn't have one, the external device must at least supply the termination power in order to feed an additional passive one, also connected to the external side".

[androda]

As an alternative, have you considered adding a termination power mod to the system?
https://www.exxosforum.co.uk/forum/viewtopic.php?t=4933

This would step around the issue entirely, and provide termination power for the external bus.

[mikrosk]

I have, yes. But since having connected BlueSCSI all the time seemed like a much easier way to go forward, I wanted to check that way first. :-)