Malfunctioning VT240 - help please

Matt Burke matt at
Mon Jun 8 03:37:49 CDT 2020

On 07/06/2020 18:27, Charles via cctalk wrote:
> Until a few minutes ago, my VT240 was operating normally, but now it's
> unresponsive (fails during power-on self test).
> Normal behavior was: display a checkerboard, then two different
> intensity all-white bands growing slowly up from the bottom of the
> screen, then a beep and the expected "VT240 Monitor Error 9" (because
> I'm using an old B&W composite monitor instead of the DEC VR201 with
> special cable). Thereafter, normal operation.
> Now, it briefly displays the checkerboard (and all four keyboard
> lights turn on, then off); then the Lock and Wait lights come on and
> nothing else happens. Blank screen.

The keyboard LEDs are used to indicate the test being performed, in this
case test 5 failed. They are listed in the pocket service guide. For the
first 6 codes it just says fatal error, replace terminal controller
board. I don't think it is the keyboard. You should get an error code 7
on screen for that.

I'm afraid I haven't looked into the VT240 firmware but I have looked at
the VT220 previously. Here are some of my notes on the self test:

0 = 8051 Internal RAM Test (Data=0xAA)
0 = 8051 Internal RAM Test (Data=0x55)
0 = 8051 Internal RAM Test (Data=0x00)
1 = 8051 Internal ROM Checksum (8840: lcall $8870)
2 = External ROM Checksum 0x8000 (884B: lcall $8870)
3 = External ROM Checksum 0xC000
8 = Screen RAM 0 (Addr=0xA000 Data=0xAA)
9 = Screen RAM 1 (Addr=0xA800 Data=0xAA)
A = Atrrib RAM 0 (Addr=0x8000 Data=0xAA)
B = Attrib RAM 1 (Addr=0x8800 Data=0xAA)
8 = Screen RAM 0 (Addr=0xA000 Data=0x55)
9 = Screen RAM 1 (Addr=0xA800 Data=0x55)
A = Atrrib RAM 0 (Addr=0x8000 Data=0x55)
B = Attrib RAM 1 (Addr=0x8800 Data=0x55)
8 = Screen RAM 0 (Addr=0xA000 Data=??)
9 = Screen RAM 1 (Addr=0xA800 Data=??)
A = Atrrib RAM 0 (Addr=0x8000 Data=??)
B = Attrib RAM 1 (Addr=0x8800 Data=??)
C = Character ROM (read only)
D = Alt Character RAM (read/write)
E = Alt Character RAM (?)

I know the VT240 uses a different processor to the VT220 but this gives
you an idea of the sort of things that are being tested early on. I
imagine the VT240 will carry out a similar set of tests.


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