How could I implement a serial communication protocol in a object-oriented form?

Question

I’m working on with an embedded device that is connected to PC using RS232.

I need to do a software to communicate with this embedded device.

I program in Delphi. I never used to use object-oriented in the past. But I'm trying to change this.

I am not able to think in an object oriented manner to solve this problem.

I have this protocol:

<STX><STX><COMMAND>[<DATA><DATA>...]<CHKSUM><ETX>

where:

<STX> is the Start of TeXt (0x55);
<COMMAND> can be 0x01 for read, 0x02 for write, etc;
<DATA> is any value;
<CHKSUM> is the checksum;
<ETX> is the End of TeXt (0x04).

The software computer will send a command via serial, and the device will answer, using the same protocol.

For example:

Reset command
PC sends     : <STX><STX><0x09><0x00><CHKSUM><ETX>
Device answer: <STX><STX><0x09><0x00><CHKSUM><ETX>

Get Version
PC sends     : <STX><STX><0x00><0x02><CHKSUM><ETX>
Device answer: <STX><STX><0x00><0x00><VER_L><VER_H><CHKSUM><ETX>

I have to send a file stream to the device.

I'd like to obtain suggestions and/or examples of the best way to implement this in an object oriented manner. I'd like to be able to do unit-test too.

Thanks

Answer 1

You should look at other serial send/receive communication models, such as HTTP. In .NET, the HTTPWebRequest object is where you gather together all the pieces of info to be sent across the wire - including command (HTTP METHOD: GET, PUT, POST, etc) and byte stream. The HTTPWebRequest object (and the HTTP stack) internally deals with the "paperwork" of calculating checksums of the data, chunking large data into smaller packets, etc. All your code has to do is construct the request object, set the command, assign a stream of data to the request object's property, and send.

Another reason why you should look at existing communication object models like .NET HTTP is that serial comms are generally asynchronous from the perspective of your host CPU. A lot of CPU time can pass by while transmitting the characters of the request on the serial port, and while waiting for the response. Use an async model for your request/response so that you don't block the calling thread and potentially freeze your UI.

To continue the .NET HTTP example, HTTPWebRequest has a GetResponse method which sends the request and will block the calling thread until a response is received. HTTPWebRequest also has a BeginGetResponse()/EndGetResponse() pair so that you can send the request and provide a callback to be executed when the response arrives at some later time.

Even if your immediate design is ok with a thread-blocking synchronous call model, you should at least investigate asynchronous coding patterns and consider implementing your object as such. You can always call an asynchronous method in a thread-blocking synchronous way, but it's much more difficult to call a synchronous method in an asynchronous way. Invest a little time now to give yourself more options down the road.