How do I know what to send to the Serial Port?

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Hi
I do know how to Open, Read, Write and Close a serial port using MFC/C++.

The problem is I do not know what to send to the serial port.

I have seen my colleagues sending something like 21 04 07 00 08 00 0F 22.

I converted it to control and it says ! EOT BEL NUL BS NUL SI "

Which also means ! (end of text)(bell)(null)(backspace)(null)(shift in)"

I'm not sure if what I have translated would be useful but I'm trying to understand what my have colleague sent and perhaps I could devise my own lines of hexadecimal to send to the port.

I get that this is a silly question but I am relatively new to this.

I was exposed to MFC/C++ and binary last week only.

Should I be unclear in my question or I am asking the question in a wrong way, please enlighten me.

I am ready to learn and I am not asking for direct answers.

I hope my fellow programming seniors in Code Project can help me out here.

Many thanks.

Posted 21-Aug-14 17:27pm

Bryan Se To

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Solution 1

What you have to send is fully defined by what you have on the other end of your RS-232 cable. Read the documentation on the device connected to it. The content is not all; you also need to know the serial port parameters: baud rate, number of control bits, handshake and so on.

—SA

Posted 21-Aug-14 19:19pm

Sergey Alexandrovich Kryukov

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Bryan Se To 22-Aug-14 3:55am

Hi.

I am unclear about the 'documentation on the device connected to it' part. May I ask what do I need to understand?

As to the parameters of the serial port, I have fully grasp it with some research. Thanks!

Are they also any keywords I could Google to learn about this problem that I am facing?

Sergey Alexandrovich Kryukov 22-Aug-14 11:15am

What keywords? You cannot Google like "find me that-I-don't-know-what". You did not tell us a single word about what you have connected.
—SA

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Bryan Se To · Accepted Answer · 2014-08-28T16:01:00

21 04 07 00 08 00 0F 22

0x21 is the start byte, the device uses it to synchronise itself in order to detect the start of the frame.

0x22 is the end byte, a confirmation that no more bytes are expected to follow. The device uses this to know that the frame was received in full so it can process the data.

0x04 is the length byte which encodes how many bytes to follow.

0x07 and 0x08 are probably 16-bit values.

0x00 could be part of the message that is stated in the protocol.

0x0F is the checksum to verify that the data is not corrupted.

The hexadecimal 0x07, 0x00, 0x08, 0x00 are quite difficult to tell what they are trying to say without looking at the documentation and protocol of the device. Sergey Alexandrovich Kryukov was right when I am required to referred to the documentation. However, there are a ton more things that I had to look at and consider. Ultimately, many thanks to Hans Passant who helped me on Stack Over Flow.