Strange issue writing to SerialPort (C#)... Works after running another program

Question

I'm trying to write to an LCD Display using System.IO.Ports in C#, with the SerialPort class.

Firstly, it works! My program does write to it successfully and it displays fine... but only after I've run a prior commerical program (LCDC).

If I run by program before running LCDC (a full LCD-display GUI program) - the LCD display sits there blankly, not even a flicker.

If I 'Start Display' on LCDC (i.e. make it display even a single screen from LCDC's catalogue) and then 'Stop Display' and shut down LCDC completely; and THEN run my program it works absolutely fine.

So it seems LCDC is activating or initialising the COM/serial port or something, that the port then remembers after the program has shut down. Does anyone have any idea?

I have tried countless code samples using SerialPort (from various online resources) and they all have the same problem - nothing before LCDC, but full functionality afterwards.

I can post code if you need but it's the standard: declare SerialPort port

port.Open( x, y z); port.Write("string");

etc. and it works fine after LCDC is run!

As an update, I'm still stuck! I used PortMon to analyse the port, firstly for my program running alone:

0   0.04459980  MatOrbOfficialT IRP_MJ_CREATE   VCP0    SUCCESS Options: Open   
1   0.00000531  MatOrbOfficialT IOCTL_SERIAL_GET_PROPERTIES VCP0    SUCCESS     
2   0.00000335  MatOrbOfficialT IOCTL_SERIAL_GET_MODEMSTATUS    VCP0    SUCCESS     
3   0.00000251  MatOrbOfficialT IOCTL_SERIAL_GET_BAUD_RATE  VCP0    SUCCESS     
4   0.00000251  MatOrbOfficialT IOCTL_SERIAL_GET_LINE_CONTROL   VCP0    SUCCESS     
5   0.00000223  MatOrbOfficialT IOCTL_SERIAL_GET_CHARS  VCP0    SUCCESS     
6   0.00000251  MatOrbOfficialT IOCTL_SERIAL_GET_HANDFLOW   VCP0    SUCCESS     
7   0.00000307  MatOrbOfficialT IOCTL_SERIAL_GET_BAUD_RATE  VCP0    SUCCESS     
8   0.00000223  MatOrbOfficialT IOCTL_SERIAL_GET_LINE_CONTROL   VCP0    SUCCESS     
9   0.00000196  MatOrbOfficialT IOCTL_SERIAL_GET_CHARS  VCP0    SUCCESS     
10  0.00000196  MatOrbOfficialT IOCTL_SERIAL_GET_HANDFLOW   VCP0    SUCCESS     
11  0.00216620  MatOrbOfficialT IOCTL_SERIAL_SET_BAUD_RATE  VCP0    SUCCESS Rate: 19200 
12  0.00295484  MatOrbOfficialT IOCTL_SERIAL_SET_RTS    VCP0    SUCCESS     
13  0.00297244  MatOrbOfficialT IOCTL_SERIAL_CLR_DTR    VCP0    SUCCESS     
14  0.00297496  MatOrbOfficialT IOCTL_SERIAL_SET_LINE_CONTROL   VCP0    SUCCESS StopBits: 1 Parity: NONE WordLength: 8  
15  0.00000251  MatOrbOfficialT IOCTL_SERIAL_SET_CHAR   VCP0    SUCCESS EOF:1a ERR:0 BRK:0 EVT:1a XON:11 XOFF:13    
16  0.00501293  MatOrbOfficialT IOCTL_SERIAL_SET_HANDFLOW   VCP0    SUCCESS Shake:0 Replace:40 XonLimit:1024 XoffLimit:1024 
17  0.00000251  MatOrbOfficialT IOCTL_SERIAL_GET_BAUD_RATE  VCP0    SUCCESS     
18  0.00000223  MatOrbOfficialT IOCTL_SERIAL_GET_LINE_CONTROL   VCP0    SUCCESS     
19  0.00000223  MatOrbOfficialT IOCTL_SERIAL_GET_CHARS  VCP0    SUCCESS     
20  0.00000223  MatOrbOfficialT IOCTL_SERIAL_GET_HANDFLOW   VCP0    SUCCESS     
21  0.00282829  MatOrbOfficialT IOCTL_SERIAL_SET_BAUD_RATE  VCP0    SUCCESS Rate: 19200 
22  0.00293892  MatOrbOfficialT IOCTL_SERIAL_SET_RTS    VCP0    SUCCESS     
23  0.00296909  MatOrbOfficialT IOCTL_SERIAL_CLR_DTR    VCP0    SUCCESS     
24  0.00297719  MatOrbOfficialT IOCTL_SERIAL_SET_LINE_CONTROL   VCP0    SUCCESS StopBits: 1 Parity: NONE WordLength: 8  
25  0.00000279  MatOrbOfficialT IOCTL_SERIAL_SET_CHAR   VCP0    SUCCESS EOF:1a ERR:0 BRK:0 EVT:1a XON:11 XOFF:13    
26  0.00294954  MatOrbOfficialT IOCTL_SERIAL_SET_HANDFLOW   VCP0    SUCCESS Shake:0 Replace:40 XonLimit:1024 XoffLimit:1024 
27  0.00288919  MatOrbOfficialT IOCTL_SERIAL_CLR_DTR    VCP0    SUCCESS     
28  0.00000447  MatOrbOfficialT IOCTL_SERIAL_GET_BAUD_RATE  VCP0    SUCCESS     
29  0.00000223  MatOrbOfficialT IOCTL_SERIAL_GET_LINE_CONTROL   VCP0    SUCCESS     
30  0.00000251  MatOrbOfficialT IOCTL_SERIAL_GET_CHARS  VCP0    SUCCESS     
31  0.00000223  MatOrbOfficialT IOCTL_SERIAL_GET_HANDFLOW   VCP0    SUCCESS     
32  0.00285763  MatOrbOfficialT IOCTL_SERIAL_SET_BAUD_RATE  VCP0    SUCCESS Rate: 19200 
33  0.00296406  MatOrbOfficialT IOCTL_SERIAL_CLR_RTS    VCP0    SUCCESS     
34  0.00298753  MatOrbOfficialT IOCTL_SERIAL_CLR_DTR    VCP0    SUCCESS     
35  0.00292132  MatOrbOfficialT IOCTL_SERIAL_SET_LINE_CONTROL   VCP0    SUCCESS StopBits: 1 Parity: NONE WordLength: 8  
36  0.00000559  MatOrbOfficialT IOCTL_SERIAL_SET_CHAR   VCP0    SUCCESS EOF:1a ERR:0 BRK:0 EVT:1a XON:11 XOFF:13    
37  0.00491962  MatOrbOfficialT IOCTL_SERIAL_SET_HANDFLOW   VCP0    SUCCESS Shake:0 Replace:0 XonLimit:1024 XoffLimit:1024  
38  0.00292942  MatOrbOfficialT IOCTL_SERIAL_CLR_RTS    VCP0    SUCCESS     
39  0.00000475  MatOrbOfficialT IOCTL_SERIAL_SET_TIMEOUTS   VCP0    SUCCESS RI:-1 RM:-1 RC:-2 WM:0 WC:0 
40  0.00245674  MatOrbOfficialT IOCTL_SERIAL_SET_WAIT_MASK  VCP0    SUCCESS Mask: RXCHAR RXFLAG CTS DSR RLSD BRK ERR RING   
41  0.00000670  MatOrbOfficialT IOCTL_SERIAL_SET_QUEUE_SIZE VCP0    SUCCESS InSize: 4096 OutSize: 2048  
42  0.00000000  MatOrbOfficialT IOCTL_SERIAL_WAIT_ON_MASK   VCP0            
43  0.00049811  MatOrbOfficialT IRP_MJ_WRITE    VCP0    SUCCESS Length 2: .X

Then for LCDC:

0   0.05153979  LCDC.exe    IRP_MJ_CREATE   VCP0    SUCCESS Options: Open   
1   0.00000503  LCDC.exe    IOCTL_SERIAL_SET_QUEUE_SIZE VCP0    SUCCESS InSize: 1024 OutSize: 1024  
2   0.00000251  LCDC.exe    IOCTL_SERIAL_GET_BAUD_RATE  VCP0    SUCCESS     
3   0.00000279  LCDC.exe    IOCTL_SERIAL_GET_LINE_CONTROL   VCP0    SUCCESS     
4   0.00000223  LCDC.exe    IOCTL_SERIAL_GET_CHARS  VCP0    SUCCESS     
5   0.00000251  LCDC.exe    IOCTL_SERIAL_GET_HANDFLOW   VCP0    SUCCESS     
6   0.00274029  LCDC.exe    IOCTL_SERIAL_SET_BAUD_RATE  VCP0    SUCCESS Rate: 19200 
7   0.00299563  LCDC.exe    IOCTL_SERIAL_CLR_RTS    VCP0    SUCCESS     
8   0.00295205  LCDC.exe    IOCTL_SERIAL_CLR_DTR    VCP0    SUCCESS     
9   0.00289757  LCDC.exe    IOCTL_SERIAL_SET_LINE_CONTROL   VCP0    SUCCESS StopBits: 1 Parity: NONE WordLength: 8  
10  0.00000531  LCDC.exe    IOCTL_SERIAL_SET_CHAR   VCP0    SUCCESS EOF:b0 ERR:1 BRK:1 EVT:0 XON:79 XOFF:13 
11  0.00296155  LCDC.exe    IOCTL_SERIAL_SET_HANDFLOW   VCP0    SUCCESS Shake:0 Replace:0 XonLimit:256 XoffLimit:256    
12  0.00000307  LCDC.exe    IOCTL_SERIAL_SET_TIMEOUTS   VCP0    SUCCESS RI:-1 RM:0 RC:0 WM:100 WC:1000  
13  0.00251401  LCDC.exe    IOCTL_SERIAL_SET_WAIT_MASK  VCP0    SUCCESS Mask: RXCHAR ERR    
14  0.00000335  LCDC.exe    IOCTL_SERIAL_GET_MODEMSTATUS    VCP0    SUCCESS     
15  0.00000251  LCDC.exe    IOCTL_SERIAL_GET_MODEMSTATUS    VCP0    SUCCESS     
16  0.00000251  LCDC.exe    IOCTL_SERIAL_GET_MODEMSTATUS    VCP0    SUCCESS     
17  0.00000391  LCDC.exe    IOCTL_SERIAL_PURGE  VCP0    SUCCESS Purge: TXCLEAR RXCLEAR  
18  0.00066545  LCDC.exe    IRP_MJ_WRITE    VCP0    SUCCESS Length 3: .P.   
19  3.94750981  LCDC.exe    IOCTL_SERIAL_WAIT_ON_MASK   VCP0    SUCCESS     
20  0.00091632  LCDC.exe    IRP_MJ_WRITE    VCP0    SUCCESS Length 3: ...

Any ideas?

I noticed the LCDC did an IOCTL_SERIAL_PURGE right before writing, but my program does this on disconnection - and then it still doesn't work after reconnecting (so I'm assuming that it's not to do with this purge).

If I run my program again after LCDC, it reads exactly the same except for:

26 0.00294954 MatOrbOfficialT IOCTL_SERIAL_SET_HANDFLOW VCP0 SUCCESS Shake:0 Replace:0 XonLimit:1024 XoffLimit:1024

Where is says Replace:0 used to read Replace:40, is this significant?

As far as I can tell... all other values set by LCDC are reset by my program after I run it, so surely it should make no difference that some values are different? Given that my program does write successfully... Only after LCDC has done it's business!

score 2 · Answer 1 · answered Jul 30 '09 at 15:36

I would think first that is not something that is getting configured on the serial port on your computer, but rather some initialization that the LCD requires before you start sending data.

I would recommend you run a tracer on the serial port to see what the other application is sending to the LCD before it sends the first line of text to display.

You could try Portmon from Sysinternals (now part of Microsoft).

score 1 · Answer 2 · answered Jul 30 '09 at 15:58

1

Make sure your handshake matches what the other end is expecting. Try setting DTR and RTS to true.

sPort.Dtr = true;
sPort.Rts = true;

answered Jul 30 '09 at 15:58

Rob Elliott

1,998
19
25

score 1 · Answer 3 · answered Jul 30 '09 at 16:11

I went through an almost identical issue recently with serial port communications. I would try connecting to my device, and no communications would work properly. Then I would open up a serial port monitoring program, to see if I was indeed getting any data transfer, and everything would look okay. Finally, I would try connecting again and I would be able to communicate properly.

What was happening in my case was that I was not setting up the baud rate, read/write timeouts, and data characteristics (data bits, stop bits, parity bits) in my program. The debug program did that automatically, and those changes seemed to "stick with" the serial port, which was why my program would work after running the debug program once.

Make sure you've set all of those settings properly in your program.

score 0 · Answer 4 · answered Jul 30 '09 at 15:38

I've had similar issues with connecting to a motor, where running another application first caused my application to work.

The issue ended up being noise on the serial port, both when writing and reading. Sending test/junk messages, and clearing the Input and Output buffers eventually cleared the issue up. In my case, the other application was just clearing the junk out of the buffers.

Strange issue writing to SerialPort (C#)... Works after running another program

4 Answers4