Brooks Instrument XacTorr CMX160 - Manual (Page 13)

p.7
Programming is accomplished through a separate control and monitoring device
plugged into the RS-485/2.5 mm serial Diagnostics port (DIAG) connection
located on the top of the CMX (See Figure 6). The 2.5 mm serial port connection
communicates through a proprietary Brooks communication protocol at 9600
baud. The following information can be read from the CMX for each relay:
• Setpoint value
• Hysteresis value
• Direction of operation
• Relay status (actuated, de-actuated)
Figure 6: Typical CMX User Software Connection
The programmable configuration is stored internally in non-volatile memory and
restored to working memory upon application of power or reset of the CMX.
2.6 ZERO OUTPUT BIAS CONTROL
NOTE: The following method for setting the Output Bias is based on using the CMX
User Software Graphical User Interface (GUI). If the Software is not available, call a
Brooks Technical Service representative for assistance.
In some applications, the process tool is not able to achieve a base pressure low
enough to correctly zero the gauge. Typically, CMXs should be zeroed at a base
pressure five decades below the gauge full-scale range. As an example, a 10T
CMX should be zeroed at a chamber pressure of 1x10
-4
Torr.
If the chamber cannot be pumped down to the appropriate base pressure, use
the Bias Adjust Control to set the desired output pressure signal.
The Zero Output Bias Control allows you to add or subtract a bias voltage to the
pressure output signal as shown in Figure 7. The units for the Output Bias
adjustment are in mV and have a maximum adjustment capability of ±250 mV.
Procedure for Using the CMX Output Bias Adjustment:
1. Pump the chamber down to the minimum achievable pressure.
2. Determine the actual chamber pressure using an appropriate method.
3. DO NOT press the Zero Button on the gauge. (Doing so will cause the CMX to
attempt to zero the gauge. If the chamber pressure is too high, the automatic
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