The following article by Hans-Christof Tuchen was previously published in ABBUC Magazine number 50.

XEP 80 - Monochrome monitor

In the summer of 1997, the ABBUC offered brand new 80 character upgrades for the price of only DM10.00(!) [about Euro 5.11 or $4.08]. These come with their own power adaptor, a connector for CVBS (composite video) monitors and a DB25 printer port. Two joystickpins are used to communicate with the computer. If this didn't happen at the unusual baud rate of 15.7 kHz, one could also use this mini terminal with other computers using an RS232 interface at TTL level. (The documentation mentions the use of different baud rates, but not how these are chosen - maybe one of the undocumented reserved commands will work.)

The 25 x 80 characters of the XEP80 screen are 11 pixels high - I have to adjust my CVBS monitor to see all of the screen. Since a TTL monochrome monitor (as used with Hercules and MDA cards) give a better picture, I hooked up one of these. That way I could also use the intensity signal, that isn't normally used in the XEP80. (The signal can be found on pin 25 of the NS405 chip.) The schematic shows the buffering of this signal by a 74LS86 chip. This one is soldered on top of the already present 7486, so that only pins 4, 7, 10 and 14 make contact. Both synchronisation signals are already present on the 7486; the video signal is grabbed from the 220 Ohm resistor just to the north, the unused intensity signal from the chip itself (actually from it's socket: you shouldn't treat the chip to your soldering iron). If you connect the second gate input to a switch, it becomes possible to invert the signal if needed. The outputs can be connected to a 9 pin subD connector (female). Via a diode and a 330 Ohm resistor, it is possible to feed the intensity signal back into the CVBS signal, but the effect is somewhat limited. The intensity signal is activated by bit 1 of the attribute register (ATRIBUTE.BAS). I also tried two RGB monitors (CGA). Since their screens are only 200 pixels high, even more lines are missing at the top and on the bottom - can't use these.

Hans-Christof Tuchen