serial peripheral added. Adjustment is affected by
VR1. Clockwise rotation reduces the clock speed. With
a printer attached via the RS232 or 20mA outputs, a
short test program may be written that will output
continual text in the form ‘1234567890123.....’ etc.
Adjustment is made by observing the printed output;
If the clock is too fast, random garbage will appear, thus:
123c5u789,P234+z7 .......... etc.
If the clock is too slow, characters will be missed, thus:
1235679013457891 .......... etc.
Note that VR1 is a multiturn (20 turns) preset, and that
the end stops are detected by an increase in rotational
torque at the ends of the track. No harm can be done by
over ‘turning’ the preset.
Correct adjustment is the mid point between garbage and
missing characters, this is a latitude of 4 to 5 turns of
The clock speed may be changed to 4800Hz (30 chars./sec.)
by changing C12 to a 8n2 10% polyester capacitor.
Setting of the 4800Hz clock is as above.
“Snow Plough” and NMI “Break”.
The snow plough is used in conjunction with IC11 to increase
the VDUSEL blanking time to eliminate ‘snow’ on the screen
during memory access to the video RAM. See Fig. 1. The
simplest method of construction is to take one of the spare
16 pin dil plugs (supplied) and cut off two pins, making
it a 14 pin plug. Then cut a piece 0.1″ pitch vero board
about 1″ wide by about 1.5″ long, with the tracks in the
longer direction. Mount a 14 pin socket at one end
(breaking tracks as appropriate) and build the LS123
circuit at the other, connecting the output of the 123 to
pin 5 of the 14 pin socket. Then solder the 14 pin plug
pin for pin to the underside of the 14 pin socket (except
pin 5). Link pin 5 of the plug to the input of the 123
circuit, and low!! a little plug in module which carries
ICll and the 123, with a plug that fits directly into IC1l
socket on the board. Neat, tidy and effective. Don’t
forget to connect power to the 123, in parallel with
pins 7 and 14 of ICll.
Plug in the module, and a TV display should appear as
usual. Tab from 0 to FFFF and adjust the preset pot
such that the ‘snow’ just disappears.