20 Oct
2005
20 Oct
'05
11:05 a.m.
On 10/20/2005 at 1:33 PM Cini, Richard wrote:
Regarding the drive termination, I distinctly remember
pulling terminators
on the middle drive in a two-drive setup...just like a SCSI chain. I don't
know about new drives (because I don't usually have multiple drives in my
machines now), but some more modern Teac drives have a termination jumper
rather than a resistor DIP pack.
You're right--I went and checked my O&A tech reference and the 5.25" drive
did indeed have a 150 ohm pullup terminator (RP1 on the schematic). I
wonder how many people adding a drive just left them on? While not
exactly within design specs (48 ma per gate), a 7438 section on the
controller should be able to drive 66 ma (75 ohm to +5) load without too
much trouble. And the drives were close enough together on the cable that
one didn't have to worry about reflections too much.
What I find curious is the use of 150 ohms as a pullup. IIRC, the MFM hard
drive, which had much trickier signal timing used a 220/330 terminator pack
(220 to +5, 330 to ground).
Cheers,
Chuck