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

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!