serial communication with device
显示 更早的评论
Hello. I have an instrument (konika chromameter CS100A) which is designed to communicate with a computer through the serial port. I bought a serial-to-usb converter to connect it to my macbookpro. The manual of the chromameter says to use a baud rate of 4800, even parity, 7 bits data length, 2 stop bits and a CR+LF terminator. To request a measurement I should send "MES", then read what is sent. I tried to do so with the following code:
CS100 = serial('/dev/tty.usbserial', 'BaudRate',4800, 'Parity','even',...
'DataBits',7, 'StopBits',2, 'Terminator','CR/LF');
fopen(CS100);
fprintf(CS100,'MES');
pause(5);
received = fgets(CS100);
However matlab returns:"Warning: Unexpected Warning: A timeout occurred before the Terminator was reached. "
And I receive: 'ÅÒ00'
Which in decimal is: [197,210,48,48,141,10]
So I have been puzzled by this for different reasons. Because I specified a data format of 7 bits, I shouldn't receive a character higher than 128, no? Also, the manual of the meter says that if a terminator is not sent 5 seconds after a command, or if the command is undefined, the meter will return "ER00". The terminator should be automatically sent by matlab, shouldn't it? I tried to add it after the command ('MES\r\n') but that made no difference. Matlab receives 6 bytes, which seems consistent with receiving ER00 + 2 terminators. The last character is 10, correctly corresponding to LF. And there is a difference of 13 between the characters 1 and 2, as between E and R, which made me suspect that the string should be "ER00". But then, why are characters 3 and 4 "0", as expected?
Does anyone have an idea on how to send the command to the chromameter and read its answer? Thank you very much.
Baptiste
采纳的回答
Walter Roberson
2015-11-8
It appears that the parity bit is not being stripped off.
I cannot promise that the serial interface is correctly generating the parity bits for outgoing data, and I cannot promise that MATLAB is recognizing the 141 as being CR for line termination purposes.
If you subtract 128 from all of the values greater than 128, you get ER00<CR><LF>
I do not know whose fault it is that the parity is not being properly removed, but you have to deal with the aftermath.
As a work-around:
Change the terminator to LF because LF happens to have even parity anyhow.
Take all of the input lines and pass them through a translation routine that removes the parity. This could be as simple as
rmparity = @(S) char(S - (S>128)*128);
Take the resulting string and process it as a string, such as with sscanf() or as required.
For output, do not output anything directly. Instead, format it in to a string, such as with sprintf(), and pass the resulting string through a routine that adds parity. You can fwrite() the result.
The routine to add parity could do an array lookup: have a table with 128 entries, entry #K giving the even-parity version of char(K-1) . For even parity, that table is
evenparity7 = char((0:127)+128*mod(sum(dec2bin(0:127,7)-'0',2),2).');
makevenparity = @(S) evenparity7(S+1);
2 个评论
Walter Roberson
2015-11-9
Good, it is nice to see this work out.
This difficulty is one of the problems that can be encountered when you use a USB<->Serial converter. USB is not a good technology to implement serial ports.
更多回答（0 个）
类别
在 帮助中心 和 File Exchange 中查找有关 Downloads 的更多信息
Community Treasure Hunt
Find the treasures in MATLAB Central and discover how the community can help you!
Start Hunting!
Translated by 
