Serial Port Communications Help

Maja Gajic

Hi I am working on a biomedical project at Univeristy and we need to communicate with a fetal CTG machine via its RS232 interface. To begin with I am just trying to send the initial packet to start communications and display the result on the console (Later I will have to process this data).

G Where CRC is the 16bit CCITT CRC split into two bytes

The above is what needs to be sent to the CTG to initiates coms, I am worried that I am not sending it correctly in the code below, The serial port settings are definitely correct.

using System;
using System.IO.Ports;
using System.Collections.Generic;

namespace SerialCom
{
class Program
{
//Created serial port object and initialised
static SerialPort com = new SerialPort(SerialPort.GetPortNames()[0],
1200, Parity.None, 8, StopBits.One);

static void Main(string[] args)
{

initiateDataStreaming();

Console.ReadKey();

}

private static void initiateDataStreaming()
{

//Open com port
com.Open();

// Write Line writes string and new line value to output buffer
// { , , "G" , , , 16 BIT CRC CCITT split into two bytes}
byte[] byteToSend = new byte[] {0x10,0x02,0x47,0x10,0x03,0x42,0x1F};

com.Write(byteToSend, 0, byteToSend.Length);

Console.WriteLine("Waiting for incoming data...");
Console.ReadKey();

com.DataReceived +=
new SerialDataReceivedEventHandler(com_DataReceived);
}

private static void com_DataReceived(object sender,
SerialDataReceivedEventArgs e)
{
// Show all the incoming data in the port's buffer
Console.WriteLine("Hello0"); // Checking if enter method
Console.WriteLine(com.ReadExisting());

}

}
}

When I run the console just says "Waiting for incoming data"
I cannot get the CTG to send data, I have checket the CRC a few times and have tried different CCITT codes to no avail.
If anyone is able to see what is wrong or able to help would be much appreciated, We have clinical trials ready to go in hospital that are waiting on me to get this working so any help is much appreciated!

4 Replies

Know the answer? Post it — somebody with the same question will find it here.

Vulpes15y ago
You can maintain padding with code such as the following:

int i = 0x421F;
string bin = Convert.ToString(i, 2).PadLeft(16, '0');
string byte1 = bin.Substring(0, 8);
string byte2 = bin.Substring(8);
Console.WriteLine(byte1); // 01000010
Console.WriteLine(byte2); // 00011111

However, if you're sending the data as numeric bytes, you don't need to worry about padding.

The byte 0x42 has a bit pattern of 01000010, no matter how it's expressed in code.

Are you sure you're sending the bytes in the correct order. Perhaps the least significant byte should be first?

In the Windows operating system and many other platforms, this is how a 4 byte integer is stored - in so called 'little endian' order.
Maja Gajic15y ago
Hi Vulpes , thanks for the reply.
I think there is a problem with the way I am splitting up the CRC codeword.

If I have a two byte word 0x421F I need to split into two bytes
to fit into the last two elements in the array below:

So to split up 0x421F I convert it into binary and split it into 8 bits. 01000010 | 0001111
The least significant byte is padded with 3 zeroes which are left out when converting
to hex? I know this is quiet low level but is there any way of keeping the zero padding?

Thanks for any help
Maja :)
Vulpes15y ago
I'd wire up the DataReceived event before you open the SerialPort and write the initial packet to it. The eventhandler will be called on a background thread.

You should only need one Console.ReadKey - in the Main() method, to prevent the application from terminating whilst you're waiting for a response.

private static void initiateDataStreaming()
{
// Wire up Handler
com.DataReceived +=
new SerialDataReceivedEventHandler(com_DataReceived);

//Open com port
com.Open();

// Write Line writes string and new line value to output buffer
// { , , "G" , , , 16 BIT CRC CCITT split into two bytes}
byte[] byteToSend = new byte[] {0x10,0x02,0x47,0x10,0x03,0x42,0x1F};

com.Write(byteToSend, 0, byteToSend.Length);

Console.WriteLine("Waiting for incoming data...");
}
Sam Hobbs15y ago
Serial communications is complicated. There were multiple ways of doing things so it can be confusing. One of the most complicated things is synchronization of the two ends. There is something called flow control that can be hardware and/or software. Your program needs to respond when data is coming in but not block in a manner that prevents the data from being received. Your program can't send the data too fast. You need to spend some time learning about serial communications. Be sure to look at the articles in this web site.

Be sure to set the baud rate correctly. As I recall, you also need to specify the number of bits (8 or 7 usually), parity (usually N or E) and the number of stop bits (usuallly 1). In addition to that, be sure to get the flow control correct.