Tech Note 105
This tech note describes how to diagnose a Rapidlogger display that shows nothing, or stops part way, when the unit is powered on. It covers both the old style monochrome LCD and the colour TFT LCD fitted to Standard units built after 1 January 2026 and to all Max, Mini, SE and ExD units, and it is the companion to Tech Note 47, the full troubleshooting guide for the old style LCD.
Two Types of Display
Rapidlogger units have been built with two different displays. The checks differ, so first identify which one the unit has.
| Old style LCD | TFT LCD | |
|---|---|---|
| Units | Standard units built before 2026 | Standard units built after 1 January 2026, older Standard units fitted with the retrofit, and all Max, Mini, SE and ExD units |
| Appearance | Monochrome, blue or green background, text only | Full colour |
| At power-on | Backlight comes on, then text | Rapidlogger logo first, then text |
| Contrast | Adjustable: press HOME, then UP or DOWN | Not adjustable |
| Connection | Two flat flex cables direct to the main board | One serial cable (TX, RX, GND, 5 V) to a separate LCD circuit board behind the display |
The Rapidlogger Max, Mini, SE and ExD have always had the TFT. A TFT retrofit is available for older Standard units. Contact Rapidlogger with the unit serial number (Tech Note 2).
What a Normal Power-On Looks Like
On either display the unit takes about 10 to 20 seconds from power-on to the first line of text. The old style LCD lights its backlight at once and may show a brief flash of random characters while the processor starts; this is normal if it clears. The TFT shows the Rapidlogger logo within a few seconds and holds it until the processor has booted. Wait a full 30 seconds before deciding that the display has a problem.
First Checks for Either Display
- Confirm that power is reaching the unit: 10 to 30 VDC at the power input terminals, or the AC supply switched on. A unit that is running shows lit LEDs on the main board.
- Check the main fuse (Tech Note 32). If only LED1 is lit on the main board the fuse is blown.
- Find out whether the processor has booted. From a computer on the unit's network, ping 192.168.0.5 or connect with RapidVu. If the unit answers, the processor is running and the job is being recorded; the fault is in the display, its cable or its power. If the unit does not answer, the problem is in the main board, not the display.
- Power the unit off, wait ten seconds, power it on again and watch the display for 30 seconds.
NOTE: The Rapidlogger records data and runs its outputs whether or not the display works. A unit that answers on the network can be left in service while the display is repaired.
Old Style LCD
This display, with its two flex cables to the main board, is found only on Standard units built before 2026. Tech Note 47 is the full troubleshooting guide for it. The table below lists the power-on symptoms and where they lead.
| Symptom | Likely cause | Action |
|---|---|---|
| No backlight, no text | Fuse blown or no power | Tech Note 32, then the First Checks above |
| Backlight on, no text | Contrast set too light or too dark | Press HOME, then UP or DOWN to adjust. Look at the screen in a dark room and from a low angle to find faint text (Tech Note 47) |
| Half of the screen blank, or lines missing | Right side flex cable not seated, or the LCD failing | Reseat the right side cable at J18 (old boards) or J15 (next generation boards), Tech Note 47 Advanced section. If lines are still missing, replace the LCD |
| Whole screen blank, backlight on, contrast makes no difference | Left side flex cable not seated | Reseat the left side cable at J14 (old boards) or J7 (next generation boards), Tech Note 47 |
| Text slow, faint or leaving ghost images in cold weather | Normal for a monochrome LCD below freezing | No action; the display recovers as the unit warms up. Adjust contrast if needed |
| Random characters for a moment at power-on, then normal | Display starting before the processor | No action |
WARNING: Reseating the flex cables requires the unit to be opened and the CPU module and LCD bracket to be removed. This should only be done by an electronics technician on a bench, with the unit powered off.
TFT LCD
The TFT has its own circuit board behind the display. The main board sends the screen contents to it over a four wire serial cable carrying TX, RX, GND and 5 V. Two power-on faults have been seen.
| Symptom | Likely cause | Action |
|---|---|---|
| Dark screen, no backlight | No 5 V reaching the LCD circuit board: fuse, or the serial cable unplugged or damaged at either end | Tech Note 32 for the fuse. If the unit answers on the network, check the serial cable at the LCD circuit board and at the main board. If the cable is good, the LCD circuit board has failed; contact Rapidlogger |
| Logo appears but text never follows | Processor has not booted, or the serial link is broken | If the unit does not answer on the network: the main board has not booted, see Tech Note 3 (firmware) and Tech Note 44 (CPU module). If the unit answers: reseat the serial cable at both ends and check that TX and RX have not been swapped after a repair |
| Logo, then text, as expected | Normal | No action |
NOTE: The logo is generated by the LCD circuit board itself. Seeing the logo proves that the display and its 5 V supply are good; it does not prove that the main board is running.
If the Problem Is Not Resolved
Contact support@rapidlogger.com with the unit serial number (Tech Note 2), which display the unit has, exactly what appears on the screen and how many seconds after power-on, whether it happens at every power-on or only sometimes, and whether the unit answers on the network. A photo of the screen taken 30 seconds after power-on is useful.