Service manual

8
PRAC T I CAL C O NS I DERA T I O NS
Which Eprom?
The smallest Eprom currently available is the 2708. This is actually obsolete and has the
disadvantage of requiring three different supply voltages: hence, it is never used for new
designs. The next larger one is the 2516 or 2716 which, although small, nevertheless has
room for 32 banks of 40 channels! It has 11 input pins of which only six would usually be
connected to the channel switch. (Although some channel switches have more than six
connections it is normally possible to get 40 unique codes from just six of them. Since we
are going to use these switch codes merely to select the LOCATIONS in the Eprom which
will contain the ACTUAL codes for the PLL, it hardly matters what the switch codes are,
provided there are 40 which are all different.) The remaining 5 input pins, therefore, can be
connected to a band selection switch to select each bank of 40 channel codes. Up to 32
permutations are possible - far more than we would ever need! This discussion assumes a
40 position switch, by the way, but the concept is equally valid for rigs with 18, 22 or 50
positions.
The next size of Eprom is the 2532 or 2732 which has 12 input pins, giving a possible 64
banks of channels from a 40 position channel switch. The 2764 Eprom has 13 input pins
and the 27128 has 14. Finally, the 27256 has 15 inputs. Getting silly, isn't it. Who needs
room for 512 x 40 = 20480 channels when the maximum coverage of the rig is unlikely to
exceed 180 channels at 10 kHz spacing? Nevertheless, these big Eproms are becoming
cheaper than the small ones and you might wish to use them, despite the waste of "space"
inside them.
O utputs
No matter how many input pins an Eprom has, the number of output pins is always eight.
This is because every "box" or location can hold only eight binary bits (or one "byte" as it
is known). This fact puts one big limitation on the use of an Eprom. What do we do, for
instance, if we want to connect the EIGHT Eprom outputs to a PLL with NINE input pins?