When trying to identify the source of the problem, first determine if
the hardware is to blame.  Perhaps the serial ports on these PC's are
problematic. Locate some serial testing software or hardware to verify
that the serial ports  are, or are not the root of the problem.

Plugging and unplugging serial cables on the PC while the port is
active may enough to damage the serial port.

Another route is to buy a serial port card and add this to the PC and
see if the problem goes away.

Also does your code perform any error trapping.  I have also seen
programs/Dlls hang onto serial ports especially on WinNT PC's where
the port is not relinquished until the PC is rebooted.
  
IMO, the MsComm control is  fairly bulletproof and I would likely
believe that the trouble is elsewhere...either in the supporting code
or hardware.
 

google{at}lethe.org.uk wrote in message
news:...
> Hi all.
> 
> I'm currently developing a system which communicates via a standard PC
> serial port to custom test hardware.
> 
> Port is variable (Com 1 to Com whatever, set in a config file). 19200
> baud, 8N1. (also set in a config file).
> 
> I'm using MSCOMM6 (Microsoft Comm Control 6.0 as listed in the
> Components window), with Visual Basic 6 Pro. Version 8176, VBA
> 6.0.8169 (if that helps!)
> 
> The PC is an Acer F1, 2.6GHz Celeron. Don't know what mobo/chipset it
> uses, because I'm in the UK, and the PC is in Spain!
> 
> The Software:
> 
> The problem is that occasionally, the serial port will "lock up". It's
> available to be opened, you can send data, but nothing comes out (as
> far as I know), and nothing is received.
> 
> Quitting/restarting the application doesn't help.
> 
> The only thing that _does_ help is a power cycle on the PC.
> 
> In the past, I've written code in Visual C, where I appear to be using
> events to receive and process the data. This code showed exactly the
> same sort of issue.
> 
> My VB code _only_ executes code when the CommEvent is comEvReceive. I
> don't service any other events. Could this be the problem?
> 
> On to the hardware
> 
> I've never had a serial port lockup with my Laptop (HP N5271) (with
> either the VB or VC code). Previously, the C code lockups have only
> been on "cheap" PCs. The company I used to work for had similar
> problems, and their only solution was to buy Dell PCs, which seemed to
> be more resilient to serial port problems. The current VB crashes are
> on Acer F1 PCs. They're not even _that_ common, they only happen
> occasionally, but occasionally is not good enough.
> 
> Has anyone else had any issues like this? Did you ever solve them? I'm
> guessing that if they have, and had, the solution will probably be to
> "stop using MSCOMM" :-)
> 
> From my electronics background, I've had issues in the past caused by
> UART errors, making a serial port lock-up until the errors have been
> cleared. Does VB/PC Serial port have an equivalent issue?
> 
> Many thanks for any help,
> 
> Pete.
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