Display and Controls
The key features of the operator display are shown below.

The main display shows the setpoint, process variable and output for the selected loop. These parameters are displayed on the bargraphs. The process variable is also displayed numerically on the five-digit numeric display. The standard control buttons operate on the selected loop.

The alphanumeric displays provide additional information to clarify the display of the selected loop. For example, the fivecharacter display may be used to show engineering units or the setpoint, and the eight-character display may be used to show loop TAG identification.

An important feature of the T640 is the loop status display. This allows the operator to see the mode of all four control loops and whether each loop is in control.

One of the functions of the eight-character Tag display is to show the presence of alarms. Alarms may be acknowledged using the ALM pushbutton.

The INS pushbutton, optionally in conjunction with the infra-red security key, allows full engineering access to all the parameters within the system. The alphanumeric displays provide messages to make this access easy. Keys are available to give two levels of access: full and partial. Additional security is available through the use of area-coded keys.

All changes made through the front panel are logged in the T640’s E2PROM filing system.

Security features
The T640 has two relay outputs whose contacts are closed during normal operation. The contacts will open if the CPU watchdog trips or if power fails to the instrument. The relays have further functionality:

Watchdog relay can be set to open if a control database is not running. The relay can also be opened from the control strategy. An input in the T600 block provides this feature.

Alarm relay will open if any function has an alarm present with priority 11-15.