I - THE BASIS OF COMMUNICATIONS

To engage a communication between two systems (machines,humans,..), you should bring together 3 conditions:

1) same physical support

Available:: coaxial wire, optical fiber, infrared module,antenna,.... There is no doubt that you will not manage to command your tv vith optical fiber (there is no plug for it) butr with an infrared remote control as the tv includes an infrared receiver.. The interfaces to establish dialog (emitter,receiver) must be compatible to work.( same technical features).

2) same transmission type

• synchronous :The datas exchanged are synchronyzed by a clock sent on an additional canal . The receiver is the slave of the emitter and is in addition triggered by the clock. The clock period can vary even during the transmission, it doesn't matter as the receiver follows it.
  
  
• asynchronous: The datas are delimited by one or more synchronisazion tops. The first , called start stop , allows the receiver to get ready for the datas sent behind. The speed of transmission who fixed et must be the same on the reveiver and emitter , can't change during a transfert. Here, again, the receiver depend on the emitter as it must got the same speed clock to decode successfully datas.If the emitter speed is changed , the receiver must be reparametrized to match the changes..

3) same protocol

There is an agreement between the two system concerning the content to be transmitted (format of the frames). If the receiver expects to get un serial frame with a specific format, I should send to it a serial frame with the same specific format so that it will be able to understand me.. Thus, the protocol defines the rules of interpretation for a frame.

What you should undertand is that at this level , I don't mind what sort of system I want to establish the dialog. (I don't want to hear things like: does it work with an HP3x or HP4x or PC ..). All I need to know is if whether yes or not a machine got the possibility to dial. ie does this machine include an serial , infrared interface or communication port?

Two possibilities , yes or no. In this last case, you will have to built an adapation interface ; that is waht I have done with the multiusage infrared as my microcontroler is not sold with an infrared interface. If this problem is solved , it means that if I satisfy the two previous conditions (same transmission type and protocol) ....then I succeed in establishing a communication canal.

Whatever the usage you will do of this canal (video, sound,data) It is the same for me àll conditions are satisfied and the speed does not cause troubles for the protocol.

II - THE LIMITS OF THIS STUDY

I won't describe synchronous links but prefer get involved in asynchronous links who are around us every days.Through pratical examples such as RS232 serial link , infrared link of the H48s(x)/g(x)) and remote control (TV,tape recorder,hifi) protocols, I will try to expose the méthodology to establish the links between two systems incompatibles. This leads me to build a model for the data transferts (including all possibilites) and to choose adaption interfaces.This last part is however difficult for an electronic newbie but don't worry , I will explain physicaly what happens with scope pictures.

1) Universal model for the asynchronous frame

A) Frame

A frame trame is the succession of elements inside a start header , group header and a stop header.

Start header of elements Group header 1,...,n of elements Stop header of elements
|		|	|
START_TRAME CONTROL_TRAME		(n times)	CONTROL_TRAME STOP_TRAME
		|
START_ELEMn CONTROL_ELEMn DATA_ELEMn CONTROL_ELEMn STOP_DATAn

 

B) Blocks

 

As shown , every element (Start,1,2,..n,stop) can be splitted into several blocs.

These blocks can be grouped by types:

TYPE	DESCRIPTION
START	warns the receiver and prepare the analysis of the element , it is generally the first bloc of the element.
DATA	contains the datas to transfert (voice,sound ,video..)
CONTROL:	Ensure the integrity of the datas, and the organizations of datas blocks or specify the destinator
STOP	Mark the end of an Element , .optionnal if control block defines the size of this block

C) Patterns

Unfortunately , a bloc can't be transmitted as it is.It must be blown up in binary elements (value 0 or 1) I call: Pattern. For example if the DATA block DATA_ELEMn is a char, it will be represented by m=8 patterns: D0 to D7.

DATA_ELEMn | D0 D1 D2 D3 D4 D5 D6 D7

ou

DATA_ELEMn | D7 D6 D5 D4 D3 D2 D1 D0

 

There is two way to organize o block : you can start by 0 motif (the LSB, Least Significant Bit ) and by the m-1 pattern (the MSB, Most significant Bit) or the inverse. So take care of this! In both cases , the (decimal) value of a DATA_ELEM block is:

DATA_ELEMn=D0*2^0 + D1*2^1 + D2*2^2 + D3*2^3+D4*2^4+D5*2^5+D6*2^6+D7*2^7;

This can be generalized by the relation:

with m the number of pattern of the block and Valeurmotif(k), the binary value of the k pattern.

a) Definition of a pattern:

Physically, the patterns are translated into an electrical waveform which can take two tension levels depending on the pattern value. The duration of the signal is called total pattern duration. The waveform and the combination of the blocks patterns make a frame according to protocol you use. There only two data patterns for a specific protocol as a pattern can represent either 0 or 1..

To define a pattern :

#You have to name it in relation with the order, it occupies in the block.

#Precise electrical features such as:

• its start value (0 or 1),
• the (total) pattern duration,
• its duty cycle (between 0 and 1): division between the high level duration by the total duration
• its end value (0 or 1)
  

To understand this I have made for you...

A START pattern can be different from a DATA ,CONTROL, STOP pattern however keep in mind that the START pattern START is constant and a frame without DATA pattern has no sense.

b) Pattern granularity

Group of patterns inside a same block inherit from the type of the block. Each block can be organized as you wish, this is the pattern distribution principle into a block. Thus, if you want send two differents type of data in a frame, this is possible only if you define a limit between these inside the DATA block type.. The data patterns will represent as well an integer as a list ... wich will be greater in size that the data patterns in a single frame. No problem, we will transfer those data objets with multiple frames, this is the encoding process of the datas. You should consider to rebuild the full data objects at the end thanks to the decoding process. These two tasks are done done with a specific communication software relative to the choosen protocol.

c) Durations

I call the inter-frame duration, the duration between the last pattern of a frame and the first pattern of the following frame. The duration of a pattern is generaly the same between blocks.In the case of the RS232 link or HP48 IR link, it corresponds to the speed of transmission (1/(Value in Bauds)). If I set up the serial port PC at 9600 bauds, the duration of the pattern will be 1/9600=104 microsecondes.

2) Sumary

Maybe you noticed that I use the communication basis by giving my rules and notations (rule 3: talk the same language!). The important thing, I hope you understood, is that a frame is a successive group of blocks including patterns. These patterns can be electricaly represented : a piece of an electric signal containing the value of the pattern. The frame is thus the result of all "electrical" patterns stuck together, taking account the order in the block and the order of the block in the frame..

3) Testing architecture

On the picture below , I show the communication links I have studied..

Orange: the RS232 links with différents usage mouse, Irman / UIR, spécific to the pic programmer... Yellow: The infrared links such as RC5 remote control link. Only the module IR multiusage can sumulate a remote control . .The Irman module will be considered as a TV receiver. Green: the infrared HP48 link I use WINSCOPE on PC as SCOPE (VDATA) , it requires an ISA port. This is a good product I recommend you if you don't have a scope yet.. Unfortunately it don't work on linux..and an ISA port is difficult now to have on recent PCs.

CClick on the picture and discover the element's feature.