MFC, Advice on handeling protocol transfer via COM Port requested

Advice on handeling protocol transfer via COM Port requested

Author	Message
Bruce Stemplewsk #1 / 4	Advice on handeling protocol transfer via COM Port requested I am writing a program that communicates to an external device via the COM Port. The device does not support hardware handshaking but does support ACK/NAK handshaking. Data is received and sent using the following packet format: [ASCII DLE character] [1 byte packet ID] [1 byte data size] [0 to 255 databytes] [1 byte checksum] [ASCII DLE character] [ASCII ETX character] If there is a DLE character needed in the size, data or checksum then an additional DLE is added as an escape character. Right now I am sending a command to the device and then waiting a period of time for a complete packet to return. Since the length of the packet can be variable I have to wait as long as it takes the longest packet to come back from the device, This is not very efficient. Any tips on how to handle this problem? Any sample code out there?
Sat, 19 Apr 2003 12:53:31 GMT

Scott McPhillip #2 / 4	Advice on handeling protocol transfer via COM Port requested Quote: > I am writing a program that communicates to an external device via the COM > Port. The device does not support hardware handshaking but does support > ACK/NAK handshaking. > Data is received and sent using the following packet format: > [ASCII DLE character] [1 byte packet ID] [1 byte data size] [0 to 255 > databytes] [1 byte checksum] [ASCII DLE character] [ASCII ETX character] > If there is a DLE character needed in the size, data or checksum then an > additional DLE is added as an escape character. > Right now I am sending a command to the device and then waiting a period of > time for a complete packet to return. Since the length of the packet can be > variable I have to wait as long as it takes the longest packet to come back > from the device, This is not very efficient. Any tips on how to handle > this problem? Any sample code out there? SetCommTimeouts is just what you need. You can set it to return when a couple of character times have elapsed with no input, so it detects the end of message by the silence that follows. -- Scott McPhillips [VC++ MVP]
Sat, 19 Apr 2003 13:29:44 GMT

leeh.. #3 / 4	Advice on handeling protocol transfer via COM Port requested Thanks Scott, So I would set ReadIntervalTimeout and ReadFile would wait this period of time for two characters (my [ASCII DLE character] andd [1 byte packet ID]) ? Is that right? Quote: > > I am writing a program that communicates to an external device via the COM > > Port. The device does not support hardware handshaking but does support > > ACK/NAK handshaking. > > Data is received and sent using the following packet format: > > [ASCII DLE character] [1 byte packet ID] [1 byte data size] [0 to 255 > > databytes] [1 byte checksum] [ASCII DLE character] [ASCII ETX character] > > If there is a DLE character needed in the size, data or checksum then an > > additional DLE is added as an escape character. > > Right now I am sending a command to the device and then waiting a period of > > time for a complete packet to return. Since the length of the packet can be > > variable I have to wait as long as it takes the longest packet to come back > > from the device, This is not very efficient. Any tips on how to handle > > this problem? Any sample code out there? > SetCommTimeouts is just what you need. You can set it to return when a couple > of character times have elapsed with no input, so it detects the end of message > by the silence that follows. > -- > Scott McPhillips [VC++ MVP] Sent via Deja.com http://www.deja.com/ Before you buy.
Sat, 19 Apr 2003 21:29:23 GMT

Scott McPhillip #4 / 4	Advice on handeling protocol transfer via COM Port requested Quote: > Thanks Scott, > So I would set ReadIntervalTimeout and ReadFile would wait this period > of time for two characters (my [ASCII DLE character] andd [1 byte > packet ID]) ? Is that right? No. First, the function is SetCommTimeout. Its sets the time ReadFile waits while there is no input. In other words, every received character resets the timer. If no character is received for the set interval then ReadFile returns. So if you set an appropriate interval it will return after the end of a received message, independent of the message length. Think of it as an inactivity detector, not a character detector. -- Scott McPhillips [VC++ MVP]
Sun, 20 Apr 2003 10:41:33 GMT

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