Coins and cash are an irritation, better use some computation. In this case a very simple system with a central authority, namely our vending machine.
We have a vending machine with a coin acceptor and dispenser. We would like to extend this system with a bill accpetor and a (reloadable) token system. The system as a whole should also be able to simply change bills into coins for those who are afraid of token systems’ designers’ incompetence.
We did some reseach and performed some experiments with a few different coin acceptors and they all seem to have a very simmilar (electrical) interface. In order to be as flexible as possible the output side of the interface consists of pins with deliver simple pulses at full operating voltage (12, 24V or more) multiple of such “channels” as they call them exist on each device. Each channel is associated with a certain monetary ammount (0.50€, 2€ or what have you).
The input side consists of pins which are meant to be driven by an open collector (or drain) arrangement. That is the acceptor embeds a pullup with one end connected to it’s preferred logic operating voltage and the other connected to it’s internal logic and the external input pin which we control.
We do not possess the datasheets for any of the coin acceptors we have, however
for the bill acceptor destined to be used in this project we do have
documentation (to be found in the doc/ directory).
From this documentation we can accetain that the bill acceptor ought to not be much different in the manner in which we interface with it. Howerver due to a shortage of pins on the microcontrollers we have handy and would use in this project we opt to use a second, rather different and less pin lusty, interface which this device also provides. The aforementioned interface is known under the name ccTalk and is in essence merely an asyncrounous half-duplex serial interface easily produced with a duo of transistors a hand full of resistors and the universal asynchronous receiver/transmitter peripheral of our microcontroller.