3.9.3 Parameter 3 (Control Parameter)
A third parameter determines how the setpoint functions. It consists of
four digits, each of which has a value of
O
or 1 (Figure 3-4). Each digit
has a specific function associated with it, which is controlled by the
digit's value. For example, the first (left) digit determines if
parameters 1 and 2 are to be compared against current or pressure. If
the value is 0, the parameters are compared against pressure, and if 1,
against current. The previous examples have all used the pressure
mode, but this value could easily be set to 1 for ion pump current.
Mode
Selection--� -
-
0
=
Pressure
1
=
Current
8 8 8
Password Protection
0 = Not Protected
1
=
Protected
� Bake Timer Mode
(Bakeout option only)
0
=
Real Time
1 =
Heater On Time
High Voltage Interlock
0
=
On
1
=
Off
Figure 3-4. LED Display for Parameter 3 of Setpoint Option
This option is most useful where the preprogrammed current-to
pressure conversions in the DIGITEL are not appropriate for the pump
you are using. Also note that if you change this parameter, it will most
likely be necessary for you to change parameters 1 and 2 as well.
The second digit is used for password protection. When this digit is 1,
the user is required to enter a four digit number before any change can
be made to the setpoint. See Subsection 3 .10 for more information.
The third digit controls the high voltage interlock, which is required
because ion pump current decreases as pressure decreases. The
problem arises when high voltage is shut off. With no voltage applied
to the pump, the current is zero and the setpoints assume that the pump
is at an extremely good vacuum when, in fact, the pressure could be
quite high. For this reason, a high voltage interlock is invoked
whenever the third digit is 0. When high voltage is shut off, the
setpoint is shut off. Normally this is always set to 0. There may be
times, however, where this interlock gets in the way of a process
control scheme. The interlock may be defeated in this case by setting
the digit to 1. The setpoint either turns on or remains on when high
voltage is turned off.
Operation
3-15
