Now that I have played with BlueSCSI and TT a bit, I understand the issue of TT's terminators better.
In its basic configuration, TT is ready for
either an internal SCSI disk
or an external SCSI disk but never both at the same time because the terminator resistor array is in the middle.
If I want to use both sides (internal and external SCSI devices), I have to remove those three resistor arrays. That implies that both devices
must be properly terminated. Then, power sources can be tricky:
- power can come from the power connector on the device
- power can come from SCSI bus (the TERMPWR pin)
- power can come from the power connector on the device *and* can be fed to SCSI bus (i.e. being the source of TERMPWR)
- power can be fed *only* to SCSI bus (TERMPWR)
The simplest setup is that drives just accept power from their 4-pin power connector, use it for its own functionality (incl. possible termination) and that's that.
Now, how this mod comes into play? If I want just an internal disk + external BlueSCSI, there's no need for the mod, both devices power their own terminators. However, if I occasionally want to connect BlueSCSI (or any other device - CD ROM, ZIP drive, ...) without opening the TT every time, I need to have the resistors removed and the external part of the bus terminated when there is no external device connected in order to have the internal device working properly.
And this is when the mod is useful - as
@frank.lukas pointed out:
https://www.atari-forum.com/viewtopic.p ... 93#p460493 even the passive terminator (basically the device
@Steve posted above) needs +5V to work properly. So if my BlueSCSI is not currently connected, I need two things:
- +5V on the TERMPWR pin (this mod)
- an external terminator connected to the DB25 connector
Initially, I misunderstood
@frank.lukas's statement here:
https://www.atari-forum.com/viewtopic.p ... 58#p460458:
In order for terminators to work on the external SCSI port, an external SCSI device must feed term power onto the SCSI bus.
I thought he meant by the "terminators" the internal (removable) ones but no, what he meant there was without this mod you have to have yet another device on the external side which would be the source of +5V.
From
@Steve's initial post is not clear what he was trying to fix -- he ended up with removed resistors, an internal device and an external terminator but why he was so keen to remove the internal resistors in the first place is unknown because such setup (assuming an internal RaSCSI) is perfectly legal and should have worked.
