Televac^® AN 3023: Automatic Control of an MX7B/MX7M with an MX2A/MX4A

The Televac^® MX2A and MX4A are thermal conductivity active vacuum gauges used for rough vacuum measurements from approximately 1*10^-3 Torr to 1000 Torr.

The Televac^® MX7B and MX7M are cold cathode active vacuum gauges used for high and ultra-high vacuum measurements from approximately 1*10^-11 / 1*10^-8 Torr to 1*10^-2 Torr.

Cold cathode gauges should not be operated at pressures above approximately 1*10^-2 Torr. Operating the cold cathode at higher pressures can increase sputtering, oxidation, and contamination, which may lead to inaccurate readings and damage to the vacuum gauge.

The MX2A and MX4A include a Set Point 2 relay output that can be configured to switch at a specific vacuum pressure.

For this application, Set Point 2 is configured near 1*10^-2 Torr and connected to the pin-control inputs of the MX7B or MX7M. This allows the MX2A/MX4A to automatically enable the cold cathode as pressure decreases and disable it as pressure increases.

This creates a standalone active vacuum gauge system in which the MX2A/MX4A measures rough vacuum and controls the cold cathode, while the MX7B/MX7M measures high and ultra-high vacuum. The combined system can cover the pressure range from approximately 1*10^-11 / 1*10^-8 Torr to 1000 Torr.

The MX2A/MX4A Set Point 2 relay output uses two pressure settings:

• SP2L (Set Point 2 Low): Pressure at which the set point activates.
• SP2H (Set Point 2 High): Pressure at which the set point deactivates.

For automatic control of an MX7B or MX7M, the following settings are recommended:

These values provide separate turn-on and turn-off pressures for the cold cathode gauge. These settings are recommendations; your vacuum system may require different values.

To configure SP2L and SP2H from the MX2A/MX4A front panel:

1. From the measurement screen, press the SEL key twice.
2. Use the up and down arrows until the display shows SP2L.
3. Press ENT. ADJ will appear at the top of the display.
4. Use the up and down arrows to adjust the value to 0.010.
5. Press ENT to save the setting. ADJ will disappear.
6. Press the down arrow. The display should show SP2H. If necessary, use the arrow keys until SP2H is displayed.
7. Press ENT. ADJ will appear again.
8. Use the up and down arrows to adjust the value to 0.012.
9. Press ENT to save the setting. ADJ will disappear.
10. Press SEL until the measurement screen is displayed or allow the display to return automatically after the timeout.

The SP2L and SP2H settings can also be configured through RS-485 communications using the W3 command.

To configure the recommended values above, send:

*XW310021202<cr>

Where:

• X is the MX2A/MX4A address. The default is 0.
• <cr> represents a carriage return which is equivalent to pressing the enter key on your keyboard.

The MX7B/MX7M must be configured for Pin Control before it can be automatically enabled and disabled by the MX2A/MX4A relay.

To enable Pin Control from the MX7B/MX7M front panel:

1. Press SEL two times, then use the up or down arrows until Vsense Control is displayed.
2. Press ENT. ADJ will appear at the top of the display.
3. Use the up and down arrows to change the setting to Pin.
4. Press ENT to save the setting. ADJ will disappear.
5. Press SEL until the measurement screen is displayed or allow the display to return automatically after the timeout.

Pin Control can also be enabled through RS-485 communications using the W8 command.

Send:

*XW81<cr>

Where:

• X is the MX7B/MX7M address. The default is 0.
• <cr> represents a carriage return which is equivalent to pressing the enter key on your keyboard.

The MX7B/MX7M Pin Control input is controlled by pins 13 and 14. When these pins are shorted together, the cold cathode is enabled. When the connection is open, the cold cathode is disabled.

The MX2A/MX4A Set Point 2 relay connects pin 11 (SP2 COM) to pin 4 (SP2 NO) when the pressure is below the configured set point. This relay connection can therefore be used to control the MX7B/MX7M Pin Control input.

During pumpdown, when the pressure reaches 1.0*10^-2 Torr, Set Point 2 activates and the relay closes the connection between MX7B/MX7M pins 13 and 14. This enables the cold cathode.

During venting or pressure rise, when the pressure reaches 1.2*10^-2 Torr, Set Point 2 deactivates and opens the connection between pins 13 and 14. This disables the cold cathode.

Power must be supplied to both the MX2A/MX4A and MX7B/MX7M.

Depending on the application, additional connections may also be required, including:

• 0 to 10 V DC analog output connections to a PLC or other control system.
• RS-485 connections to a PLC or other control system.