🚀 go-pugleaf

Rebecca and Rowland <real-address-in-sig@flurbletigibbet.rhubarb.foobar>
wrote:

> Hang on - that can't be right.  Termination resistors don't need any
> power at all.  They connect from the signal lines to a 0V line.

SCSI2 specification 5.4.1:

All signals not defined as RESERVED, GROUND, or TERMPWR shall be
terminated at both ends of the cable.  The implementor may choose one of
the following two methods to terminate each end (see figures 9 and 10):

   a)     The termination of each signal shall consist of 220 ohms
(+/-5%) to
          the TERMPWR line and 330 ohms (+/-5%) to ground.  Using
resistors
          with +/-1% tolerance improves noise margins.
   b)     The termination of each signal shall meet these requirements:
       1)    The terminators shall each supply a characteristic
impedance
             between 100 ohms and 132 ohms. 
       2)    The terminators shall be powered by the TERMPWR line and
may
             receive additional power from other sources but shall not
require
             such additional power for proper operation (see 5.4.3).
       3)    The current available to any signal line driver shall not
exceed
             48 mA when the driver asserts the line and pulls it to 0,5
V d.c. 
             Only 44,8 mA of this current shall be available from the
two
             terminators.
       4)    The voltage on all released signal lines shall be at least
2,5 V
             d.c. when the TERMPWR line is within specified values (see
             5.4.3). 
       5)    These conditions shall be met with any legal configuration
of
             targets and initiators as long as at least one device is
             supplying TERMPWR.

The first termination method above is the same as in SCSI-1.  The second
termination method is recommended for better signal quality.