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JAM2014
Joined: 24 Apr 2014
Posts: 138
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| USB CDC oddity..... |
 Posted: Tue May 02, 2017 7:48 am |
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Hi All,
I'm experiencing an odd behavior with a couple of USB CDC devices I've got, of course built with the CCS USB code.
These devices work great individually, but when I connect two to my Windows 7 PC I observe something odd about the COM port mapping. Specifically, they seems to swap COM port numbers when both are plugged in. For example, I plug in the 1st device and check the Windows Device Manager and see this device is mapped to 'COM5'. I then plug in the 2nd device and see that a new port (COM6) shows up, so I assume this is the 2nd device. But, when I attempt to communicate with the 2nd device on COM6, the 1st device on COM5 responds, so it seems that the COM port 'mapping' is not preserved.
I assume this may happen also each time the computer is booted with both devices plugged in?
Both USB CDC devices are identical, so how can I make them unique so that this COM port oddity does not occur? I need device 1 to always be COMx, and device 2 to always be COMy.
Thanks,
Jack |
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Ttelmah
Joined: 11 Mar 2010
Posts: 19342
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| Posted: Tue May 02, 2017 8:36 am |
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If the devices are identical, you won't.
They need to have unique serial numbers. In the descriptor, there is an unused (0) entry for the serial number string descriptor. You need to change this to '3' and add a serial number to the string descriptor. If you look at the hid descriptor, this has a serial number, and shows how it has to be done.
With this done (different numbers in the two devices), if you then use the advanced option in device manager for the com port, for the first device and set it to a specific port number, then this will become associated to this device's serial number. Do the same for the second port.
With the different serial numbers, Windows can tell the devices apart. Without this, the port numbers will be assigned on 'first come first served' basis, and will change with the order the devices are plugged in, what USB ports they are on, etc. ect.. |
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JAM2014
Joined: 24 Apr 2014
Posts: 138
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| Posted: Tue May 02, 2017 11:55 am |
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Hi Ttelmah,
Yes, your description is pretty much what I was assuming was going on. I checked and it doesn't seem too difficult to add the unique serial number to my device. The HID descriptor provides a handy road map for this!
Thanks!
Jack |
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JAM2014
Joined: 24 Apr 2014
Posts: 138
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| Posted: Wed May 03, 2017 1:20 pm |
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Hi Again,
Well, I have not been able to get this working in spite of it seeming to be pretty straight-forward.
I did the following additions to usb_desc_cdc.h:
| Code: |
#if defined(USE_USB_SERIAL_NUMBER)
0x03, //index of string descriptor of serial number ==16
#else
0x00,
#endif
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to the device descriptor, and:
| Code: |
// string 3 - serial number (this is optional, but we specified it's use in the device descriptor)
18, //length of string index
USB_DESC_STRING_TYPE, //descriptor type 0x03 (STRING)
'1', 0,
'2', 0,
'3', 0,
'4', 0,
'5', 0,
'6', 0,
'7', 0,
'8', 0
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to the string descriptor.
I also added a flag to my program above the include for usb_cdc as follows:
| Code: |
#define USE_USB_SERIAL_NUMBER TRUE
#include <usb_cdc.h>
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Have I missed anything obvious?
So, the code won't compile, with all the errors occurring in the modified usb_desc_cdc.h file. Of course, the errors reference line numbers in the file, but I'm not sure how that relates to actual lines in an external source file? This is a new one for me.....
Jack |
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Ttelmah
Joined: 11 Mar 2010
Posts: 19342
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| Posted: Thu May 04, 2017 12:34 am |
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I'd guess you have probably left the comma out on the line before:
| Code: |
'U',0,
'A',0,
'R',0,
'T',0,
// string 3 - serial number (this is optional, but we specified it's use in the device descriptor)
18, //length of string index
USB_DESC_STRING_TYPE, //descriptor type 0x03 (STRING)
'1', 0,
'2', 0,
'3', 0,
'4', 0,
'5', 0,
'6', 0,
'7', 0,
'8', 0
};
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Note the comma after the 'T',0 entry.
The last element in the array always has no comma. Since you are extending the array, a comma has to be added.
I have just built the ex_usb_serial example, with these mods, and Windows happily recognises the com port, and in device manager, shows it having a serial number correctly. |
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JAM2014
Joined: 24 Apr 2014
Posts: 138
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| Posted: Thu May 04, 2017 7:45 am |
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Hi Ttelmah,
Argh! Yes, that was it, a simple missing comma  !
Thanks,
Jack |
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Ttelmah
Joined: 11 Mar 2010
Posts: 19342
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| Posted: Thu May 04, 2017 7:50 am |
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I was trying to work out what to call this.
A 'virtual' second set of eyes?.
Spotting the missing comma, when it wasn't even posted!...
Has to rate quite highly as a good guess.  |
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JAM2014
Joined: 24 Apr 2014
Posts: 138
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| Posted: Thu May 04, 2017 7:58 am |
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| Ttelmah wrote: |
Spotting the missing comma, when it wasn't even posted!...
Has to rate quite highly as a good guess. |
Absolutely no argument there! I'm glad your crystal ball wasn't out for service this week! 
Jack |
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