Service manual

47
G a ng B a ng e rs
If you are going into Eprom copying in a big way there are commercial units (known as
"gang bangers" in the trade) which will program several Eproms at a time. The simpler type
for connection to a PC will seldom program more than one device at a time, but is far more
versatile for development purposes.
M a nu a l Progr a mming
It is actually possible to enter data into an Eprom without using an automatic programming
unit at all. The method is tedious and varies according to the type of Eprom you are using.
Basically, you must connect a row of switches to the input address pins to select either 0v
or +5v. Similarly, the output Data pins must be connected to switches. Some Eproms
require a Vcc supply of +5v and others require +6v during programming. In addition, a
supply of +26v, +21v or +12.5v is required on the Vpp pin. Set the address switches to
select the location; set the Data switches to choose the contents; apply a programming pulse
to the Pgm pin for the appropriate duration (approximately 50 milliseconds for a 2716, less
for other devices). The polarity and duration of the pulse are very important and must be
chosen from the manufacturer's data. A simple pulse timing circuit would be required, in
addition to the stabilised voltage supplies. Of course, since you are using switches, the
locations and codes must be in Binary notation. It is easy to damage Eproms when
programming in this way and the manufacturer's literature should be consulted. The supply
voltage Vcc must be switched simultaneously with the voltage on pin Vpp, or else Vcc
must already be switched on at the time when Vpp is switched either on or off. If this point
is ignored you will certainly destroy the Eprom.