COM Port Handshaking

[Peter Reid]

>On Friday, April 12, 2002, at 06:32 AM, Peter Reid wrote:
>
>>Can someone tell how to set up a PC COM port for hardware or 
>>software handshaking.  Any other advice or examples of COM port 
>>working would be appreciated.
>>
>The device on the other end needs to understand the handshake. 
>Check your manual for that.  Also, if you use hardware handshaking, 
>then the cable needs to be wired for it.  The recommended cable for 
>the equipment might also provide a clue as to handshaking.

The cable is fully wired (8 lines + earthing) to support hardware 
handshaking.  Unfortunately the equipment is rather old, the manual 
rather thin on detail and no longer supported by the current 
manufacturer.  So I can't get any more detail from the original 
suppliers.  The comms settings on the equipment's physical control 
panel doesn't mention xon/xoff or any form of handshaking, just baud, 
word length (7/8 bit), stop bits & parity, line ending (CR/CR+LF), 
duplex (half/full).

>Use the serialControlString property to set up handshaking before 
>opening the COM port.
>
>For software handshaking, include "xon=on" in your string.
>
>For hardware handshaking, use one of the hardware control 
>substrings.  I have not used these yet, but the doc says it is the 
>same as the the string for the mode command in Windows.  The 
>documentation for that is slightly different in important ways.  I 
>suspect you want "rts=hs", but I'm just guessing.  I'd be pleased to 
>learn what you find out.

I've been experimenting with these, but find that sometimes only part 
of the command is received by the equipment.  All commands are 3 
character codes with optional numeric parameters. All commands are in 
7-bit ASCII.

>Some equipment cannot talk and walk at the same time.  For some you 
>should not send a command until you have the complete response from 
>the previous command.  Also, some equipment will send unsolicited 
>information and that might confuse your code.  In this case set up 
>response handling to be always on and independent of commands.

What I'm not sure of is the correct handshaking protocol and how I do 
this in Rev. How do I use CTS, DTR etc. assuming that these lines are 
being used (which the equipment manual implies that they are). Are 
there any messages associated with the COM ports that I can trigger 
on or wait on?

>ASCII data?  If the response might includes nulls, remove or change 
>those before putting the response into a field.  Also, you might 
>need to do some end-of-line transformations.

Yep, happy with this.

>Before you get too far into handshaking, you might want to look at 
>your scripts with an eye on timing.

Yep, happy with this issue as well

>Dar Scott

Thanks Dar for the feedback
-- 
Peter Reid
Reid-IT Limited, Loughborough, Leics., UK
Tel: +44 (0)1509 268843 Fax: +44 (0)870 052 7576
E-mail: preid at reidit.co.uk
Web: http://www.reidit.co.uk