THE SIMPLIFIED THEORY 

1)  The RS232 systems

      They are the combination of an  interface and  a receiver/emitter.

• A - The RS232 interfaces
•  
  • a) The Max232
    
    
  • This chip is  "magic"  because with a single power  of 5V, it generates an Rs232 signal  +9/-9V (inside there is an voltage elevator and an inversor so 4 externals capacitors of 10uF are needed).

1. b)Le  quadruple récepteur / transmetteur de ligne
   

   1. In some case, if we only need one way of communication (ie reception or emission), it is posible to use  a unidirectionnal RS232 line driver such as:

1.  

2. Line Driver	Adaptation	Function	Requires
   DS14C89	4 * RS232->TTL	Converts signals +3/-3 to 0/5V compatible TTL levels	single power 5V
   DS14C88	4* TTL->RS232	Converts signals TTL en +3/-3V min	double power + 6V# /- 5V

1. This circuits like the max232 are inverters. As far as concerned the chip   DS14C88, it needs at least 6V to works correctly.  The most easiest way   to solve this problem is to replace the chip by a  single transistor PNP shown below:

2.  

3. This gives always an  inconvenient : you should use an symetric +5/-5V power supply..   If, in the circuit, nothing requires -5V then, the good thing is to use an max232.

   
   B- RS232 Encoders/decoders 

    

   1. The link mode can be synchronous (the last mode requires an extra wire for a clock )  or asynchronous  (the frames arrive randomly and are taken into account immediatly ). As the RS232 is a seria lasyncronous link, the HP48 should ever listen his serial  input port in case of  a frame is being adressed.
   2.  
   3. There are two means of managing frames:  the active lookup (we wait a frame arriving and we do nothing else)  and the passive lookup (in this case, this could be either an external chip doing the job such as an  UART - Universal Asynchronous Receiver Transmitter or  the processor  triggered by an   Interruption  to decode the frame).
   4.  
   5. If we want to extract the 8 bits of from the serial frame our HP nicely encode into a serial frame between the start and the stop bit , we can then use a specialized chip from the UART family . Unfortunately, making these working is quite difficult  as these must be set up by software which is not accessible for common people. The idea was to find an UART which can be set up by harware ie by putting logical level on its pin to define the transfert rate, the number of data bits per frame.
   6.  
   7. I have found hhis sort of chip, see my realization  Lab48 to learn how to use it, it is the AY3-1015D (cost 150F) - It is true that it is not get one because it was the first generation of UART inside the PC keyboards. Don't worry , i have found a cheaper replacement thz PIC16C84 (64F), programmable in C through the seria liinks thanks to the fabulous  pic  programmer pip02 of  JDM.- I will expose this solution inside my project Hpdomotic.
   8.  
   9. The  principle of UART is  to  synchronize on the positive edge of the start bit , read
   10. lthe state in the middle of the 8 data bits (this requires   an clock  at least 2*Tbaud -
       into  la HP , there is a clock 16 time faster for it) and to memorize all the separated bits into a 8bit widereception buffer. The content of the buffer can be accessible  directly on 8 pins of the chip or dynamically  read by a microcontroller. 
   11.  

2)  The Infrared systems

• A -  IR Interface:

   

  • a)  Half solution  in wired logic
    
  • I manage to get  on the  intenet , an wired solution  from Tony Duel.. I have not tested it and i don't know if it works but I give  you on the  ASCII-ART form..

where the phototransistor is an Maplin YY66W (Infra Red Sensor) = TIL78
and the infrared diode  a Maplin YH70M (Infre-Red Emitter) = TIL38

In fact , in this circuit we notice an max232 who   transforms the modified    IR frame  to make it compatible to RS232. This do not solve the problem we got to decode RS232 frame either.

 

b) My  IR interface

The problem can often be solved  with a good choice of electronic components.As Hewlett Packard  is also recognized to be a great manufactuer in the domain of the optoelectronic, why not choose its products? Two years ago, I was interrested in Irda stuff.(transfert between PC <-> laptops by ir ). There are some Irda modules as well at Temic (Siemens Telefunken)  as Hp, ready to run  called the "Infrared IrdA Compliant Transceiver". I bought one of each, and hopping it will works and my dreams has became reality.

The chip  HSDL-1001 (available at Radiospares in France for 30 f ) who contains a photodiode and a infrared diode (with  UV filter) and  TTL buffers, is adapted to transmissions up to 115kbit/s with an wavelength between 850 and 900 nanometers (the infrared diodes inside of the  hp48 are optimized at 880nm ).

All the secret of this cicuit remains into the  length of the pulse . The well known chip NE555 is wired as an  retriggerable monostable : for every negative  pulse presented on  the TR pin(trigger), we get on the output  an hight pulse calibrated at :

   Tw= R3*C3* ln3

In our case , it gives : Tw=297 microseconds  who should be less than the duration between two pulses ( 416.7 microseconds). Thus, we will have the time to read correctly the level of the pulse (if there is) which is the complement of the bit sent.(0= a hight pulse at 1 level of duration  267 microseconds, 1 = no pulse then a 0 level of duration 416.7 microseconds).

Unfortunately, the chip HSDL-1001 can not be employed for the emission too.  Indeed, in the technical documentation, the maximum duration allowed for the pulse at hight state  is only  2.15 microseconds ( Irda compliance) and we need in fact  52 microseconds. Let's use then an easy transistor amplifier  (Q2= BD139) for it. 

B -  IR: Encoder / decoder 

The choice is easy  as  the chip 16C84 from Microchip  is  reprogrammable and  the tools are free. It is a 8 bit  micocontroller with internal 1024k  flash and the software can be developped with C tools.  It is easy to transform it into an serial UART  and IR decoder for the HP.

The Clock , prog and data signals are used by the programmer. People should here know than Pip02 of JDM can not programs the chip on circuit when the ground of the PC is used (Scopemeter on PC shortcuts the programmer by ground loops!!!) however,  the programmer is universal  and easy to build. By the way, big thanks to JDM.

I optimized an circuit in the french magazine  "Electronic pratique". I will give you the schematics as soon as the hpdomotics project will be finished.