Service manual

nsistor

If the codes require the 9th pin always to be a "1" or always a "0" then there is no problem.

We connect that pin to the supply and ignore it. If, however, the state of the pin must

change between "1" and "0" then we do have a problem. The answer is usually to connect

the 8th pin to the base of a transistor whose collector is connected to the ninth pin. The 9th

pin will then

always have the inverse voltage from that which is applied to the 8th pin. When the 8th pin

is "1" the 9th pin will always be "0". (The Export version Cobra 148 GTL DX used this

system with a 15.45 MHz

downmix crystal to begin with. Eventually, however, it occurred

to somebody that by using a 15.00 MHz crystal the range of codes could be shifted to make

pin 9 permanently low and the transistor was no longer needed!)

of Cod

It can be shown that this method can increase the possible number of codes by up to 128

within a particular range, but does not increase the overall number of codes possible with

Eprom, which will always be 256 due to the limitation of having only eight output pins.

Sometimes the transistor base is connected to both the 7th and the 8th pin. In this way, the

range of codes is shifted a little further and the 9th pin will be low if either the 7th pin or

8th pin or both are high. The following list of binary codes will help you to

visualise this

concept. Remember that a total of only 256 codes can be used, but the range can be moved

up or down by use of the transistor inverter.

1 0100 0000 1 0000 1010 0 1101 0100 0 1001 1110 0 0110 1000

•1 0011 1111 1 0000 1001 0 1101 0011 0 1001 1101 0 0110 0111

1 0011 1110 1 0000 1000 0 1101 0010 0 1001 1100 0 0110 0110

1 0011 1101 1 0000 0111 0 1101 0001 0 1001 1011 0 0110 0101

1 0011 1100 1 0000 0110 0 1101 0000 0 1001 1010 0 0110 0100