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VB6 - RS485 Communcations

Posted on 2000-05-09
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Last Modified: 2010-07-27
Upgrading a VB6 project from 16 bit to 32 bit.  This program will be used on NT machines(32-bit) as well as Windows 95/98.

Old program used the RS232 serial communications where no hardware handshaking took place, ie. RTS line not toggled.

The 32 bit needs to use the RS485 serial communications protocol where RTS line must be toggled, ie. hardware output buffer (UART) needs to be monitored in order to know when to toggle the RTS line.

Is there a way to check and see when the UART buffer has been emptied?  It is my understanding that the OutputBufferCount property only moniters the software's output buffer, not hardware.
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Question by:bmoser
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Expert Comment

by:Erick37
ID: 2792977
From Help:

"Setting the SThreshold property to 1 causes the MSComm control to generate the OnComm event when the transmit buffer is completely empty."

Private Sub MSComm_OnComm ()
   Select Case MSComm1.CommEvent
   ...
   Case comEvSend
      'Fired when buffer is empty (SThreshold = 1)
   'Other useful events:
   Case comEvCD 'Change in the CD line.
   Case comEvCTS 'Change in the CTS line.
   Case comEvDSR 'Change in the DSR line.
     
   End Select
End Sub
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by:bmoser
ID: 2792980
Edited text of question.
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Expert Comment

by:GivenRandy
ID: 2793150
If you use the MSComm control, it will handle handshaking.  For more detail see "Serial Port Complete":

http://www.amazon.com/exec/obidos/ASIN/0965081923/randygivennancyg/002-9129594-2973007

We have done RS-485 / 422 and RS-232 written in VB.  For quick-fixes of some legacy code, we have used existing RS-232 code with RS-485 adapters (no software changes!):

http://www.bb-elec.com/product.asp?dept%5Fid=3&sku=485DRJ 
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GivenRandy earned 50 total points
ID: 2949538
If you use the MSComm control, it will handle handshaking.  For more detail see "Serial Port Complete":

http://www.amazon.com/exec/obidos/ASIN/0965081923/randygivennancyg/002-9129594-2973007 

We have done RS-485 / 422 and RS-232 written in VB.  For quick-fixes of some legacy code, we have used existing RS-232 code with RS-485 adapters (no software changes!):

http://www.bb-elec.com/product.asp?dept%5Fid=3&sku=485DRJ 
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