STU 540 display remains blank but an "Evaluation" signature window appears on PC monitor

This situation can occur when a signature application tries to capture a signature from an STU 540 assuming that it is in HID mode, whereas in fact it is in serial mode.
The STU 540 can operate as a standard USB HID device or serially via a virtual COM port. To verify whether a 540 is in HID or serial mode there are two main options:

1. Open up Device Manager on your PC and look under "Ports (COM & LPT)" or "Universal Serial Bus Devices". If the first of these sections shows an entry for "USB Serial Port (COM5)" (where COM5 could be any COM number from COM3 upwards) then the 540 is in serial mode. For example see the screenshot below which shows two 540s, the first one in serial mode on COM5 and the second in HID mode:

2. Read the guidance on STU Serial Connection and use getSerialPorts to see if a virtual COM port is displayed as follows:

D:\SerialTestUtils>getserialports
COM5  virtual

The above indicates that the 540 is connected to virtual COM port COM5 i.e. it is operating in serial mode.

Normally 540s are delivered from the factory in HID mode but occasionally customers find that they are in serial mode.
Special configuration is required in the signature application if the 540 is to be used serially.
For more information on this please see the STU 540 Installation Guide and Uploading images to the 540

If you want to use the 540 in standard HID mode (this is the most common scenario) and the serial test utilities mentioned above indicate that it is in serial mode then you will need to use the serial2hid tool (see the STU Serial Connection link above) to convert it back to HID.

e.g. serial2hid COM4 128000