ELF Mini-Terminal Video Fix

The ELF mini-terminal developed a rather odd fault. The cursor disappeared and reverse video did not work. The CRT alignment test screen looked odd too. Investigating this issue led me to trace out some of the circuitry of the terminal.

Oscillator and Mode Selection

The 20 MHz crystal-controlled oscillator provides the clock for most of the board. A 74hc258 multiplexer is used to select between 40 and 80 column signals. In 80 column mode, the 20 MHz clock is used directly, which is quite challenging for the logic in use. A lower frequency clock is used in 40 column mode.

 

Inverse Video Logic

The 6845 CRT controller generates a CURSOR signal, which is used to invert the video signal in this case. The character generator is a SAA5055, which is intended for the now obsolete Teletext system. This is somewhat overclocked in 80-column mode, but it seems to cope. This device only uses the lower 7-bits of the RAM data, and the terminal uses the otherwise unused bit 7 to invert the video in the same way as the cursor does. There is a significant delay between RAM read data entering the SAA5055 and the associated RGB output. This requires that the invert signal be similarly delayed so the inverted characters (and the cursor) appear in the correct location. This is the purpose of the chains of flip-flops IC23 and IC28.

 

Video Output

The video supplied to the monitor is taken from the SAA5055 character generator and inverted as required. The red, green and blue signals are mixed with a software-controlled bias (the brightness defined in the terminal setup menu) before being supplied to the monitor.

The fault in this case was IC28, a 74ac374 octal latch. This inhibited the invert video signal, which I called INV_DLY2 in the diagrams above. Fortunately, these devices are still available.

The PCB has only two copper layers. The layout must have been quite a challenge, as of course this includes power and ground. In addition, the pixel clock is 20 MHz in 80-column mode, which is quite challenging for the logic families used. A slightly alarming feature is that the +12 V supply is routed to various places mixed with the logic. A momentary short-circuit could do a lot of damage.