Accessing Serial Port in Access / VBA

Hi,
I have a need to send simple text/hex values to a serial port from an Access application (which controls some relays incidentally). I've read many long articles regarding MSCOMM and variants thereof but I'm sure it shouldn't be that difficult.
From a previous answer to another question, I have arrived at the code below which is nearly there except Windows seems to be adding data at the start and end of my own.

Global Const stx = 4
Global Const etx = 15

Public Sub relay_on(n)
    cmd = 20
    mask = 2 ^ (n - 1)
    'param1 and 2 both zero
    chk = 231
    
    Open "COM3" For Output As #1
    Write #1, Chr$(stx) & Chr$(cmd) & Chr$(mask) & Chr$(0) & Chr$(0) & Chr$(chk) & Chr$(etx);
    Close #1
End Sub

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I'm using a serial port 'sniffer' and when I run the procedure shown, I see the data I'm sending but also 22h at the start and then 22h,2Ch at the end - is there a way to remove this extra data? I guess it's to do with the port communication settings but I'm not sure how to configure those within VBA.

Any help would be appreciated.