RTS/CTS and baudrate emulation
The virtual serial port driver for Windows.
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Hello,
I have tried com0com on Windows XP Pro SP2.
Basic functionality (data transfer between the two virtual ports) works fine.
Anyway it does not seem to support two features which I need sometimes.
The first one is baud rate emulation. If I use a null modem cable to connect two real serial ports, and set the two ports to run at 2400 bps, data transfer is 4 times slower than when I use 9600 bps, as expected. On the contrary, when using com0com, the baud rate setting does not seem to make a difference: data is always transferred at the same speed.
The second one is hardware flow control (RTS/CTS).
I have a test application (App_A) which receives data on a serial port, using a small input buffer, and sleeps (using the Windows API call Sleep) for 2 seconds after every receive event. I have another application (App_B) which sends data to App_A, and monitors the CTS signal. The purpose of the two apps is to verify RTS/CTS management.
When I set App_A and App_B to work at 57600bps, and try to transmit a 40 kB text file from B to A, App_A gets its buffer full before it can read all data coming from App_B, so it needs to set its RTS line to 0 to avoid buffer overflow. App_B then detects the change on its CTS line (which corresponds to A's RTS). App_A sets RTS back to 1 when it is ready to receive more data.
When I run the transfer using two real serial ports, I can see that App_B detects the changes on the CTS line when App_A's buffer is full and stops until App_A is ready to receive again. When I use the virtual com0com port pair, App_B does not detect any change in the CTS signal; so it continues to send data, and App_A has large buffer overflows (in fact at the end of transfer the received file is only 18kB instead of 40). Also, with the real serial ports App_A notifies me when there is a buffer overflow, while this does not happen with the virtual ports.
I would like to know if you plan to implement support for these features in the near future.
Thanks for your work.
I plan to implement support for these features in the future. I'm not sure that it will be in the near future.
The baud rate emulation now implemented in CVS version.
> App_B does not detect any change in the CTS signal; so it continues to send data, and App_A has large buffer overflows (in fact at the end of transfer the received file is only 18kB instead of 40).
It's a strange issue because the buffer overflow emulation is not implemented yet.The virtual port pair works like a pipe. If App_A's port gets its receive buffer full the com0com suspends write requests from App_B till App_A reads some data.So all data sent (and examined) by App_B should be received by App_A w/o lost.
>> App_B does not detect any change in the CTS signal; so it continues to send data, and App_A has large buffer overflows (in fact at the end of transfer the received file is only 18kB instead of 40).
> It's a strange issue because the buffer overflow emulation is not implemented yet.The virtual port pair works like a pipe. If App_A's port gets its receive buffer full the com0com suspends write requests from App_B till App_A reads some data.So all data sent (and examined) by App_B should be received by App_A w/o lost.
I installed v.1.3.0.0 and it behaves as you say (no more overflows). Actually after this test I tried uninstalling, rebooting and installing 1.2.0.0 again, and even with that version there were no overflows.
Don't know what was causing that behavior before. Anyway, it seems solved now.