Iran is a rather wide country, and, as a rule, the chrage per one call in
S. T .D . (Subscriber Trunk Dialing ) is required more compared with a local
call , So, just after the opening of the STD service widely in national telephone
network, unconcerned use of the S. T. D. service has been caused on an incon-
ceivable increase in telephone charge of th subscriber. This is a common problem
for all subscribers from home telephone to private or govermental offices.
Then, many subscribers requested to have a device available to control
trunk dialing. Therefore TCI (Telecommunication Company of Iran) asked us
to solve this problem specially for governmental offices .
We recommended several methods to T. C. I. for controlliug of toll dialing
such as rearrangement of wiring in telophone offices, applying the counting
method, applying the AC signalling method, and so on.
This report is the main part of our recommendation about «0» blocking
by impulse counting method.
In this article, four circuits arc presented , which have been developed
one by one in our ladoratory. So we have improved the device from the first
circuit to the next, and in some steps the circuit has been fully modified .
There are many interesting points, mentioned hereafter that we may say
they are suitable circuits. The first and second circuits have to be installed in
telephone offices .
The first circuit begins to count 1200 ms since the arrival of first dial
pulse in the first digit, but in the succee ding three circuits IS applied another
method called only first digit counting .
The third circuit should be connected to telephone line on telephone set
or PBX (Private Branch exchange), with a high parallel resistance (more than
3MEG OHMS) .
The fourth circuit employs only a very low resistance (1 to 2 OHMS) in
senes to the line , and provides two considerable features, no restriction for
connecting point of the circuit to the subscriber's line, and no using any de
current during hook on as previous one .
The consideration in those circuits were studied as follows;
1) Reasonable price
2) Small Size
3) Using a 5- Volt de power supply only
4) Wide range for acceptable speed of dial with high reliability to count
the pulse
5) Using a _very high resistance In parallel or a very low resistance In
series,
6) Using power off relay circuit and emergency battery .
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