DEC DECtape 551 Manual page 63

The +10-volt margin should be approximately ±8 volts and the -15 volt margin should be ap-
proximately ±3 volts. It is important that the -15-volt margin not be increased above -18 volts
or damage can result within the logic circuits. By recording the level of bias voltage at which
circuits fail, progressive deterioration can be plotted and expected failure dates predicted.
Therefore these checks provide a means of planned replacement.
A color-coded connector at the right side of each rack of modules, as seen from the module
side, provides connection to the normal and marginal operating voltage. Either the normal or
marginal voltage supplied to the connector is selected for application to terminals A, B, and C
of all modules in a rack by a switch at the end of the rack. Normal +10 and -15 volt lines are
permanently connected to the output of the Type 728 Power Supply. Marginal +10 and -15 volt
lines are common to all racks and are connected to the output of the Type 734 Variable Power
Supply in the central processor via the I/O bus. The color coding of these connectors is as
follows from top to bottom:
1. Green, +10 vdc internal marginal-check supply
2. Red, +10 vdc normal internal supply
3. Black, ground
4. Blue, -15 vdc normal internal supply
5. Yellow, -15 vdc external marginal-check supply
Three, single-pole, double-throw switches at the end of each rack of logic allow selection of
either the normal or the marginal-check power supply {for distribution to the logic}. The top
switch connects the +10-volt supply to terminal A of all modules in that rack. In the down
position, the fixed internal +10-volt supply {connected to the red terminal of the Type 728} is
supplied to the modules; and in the up position, the marginal-check voltage {supplied to the
green terminal from the power bus} is applied to terminal A of the modules. The center switch
performs the same selection as the top switch for application of a nominal +10-volt level to
terminal B of all modules. The bottom switch selects the -15-volt supply to be routed to ter-
minal C of all modules. In the down position, the fixed -15-volt output of the 728 Power
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