RE: [OT(ish] Duplicate Character between modems

["Neil Everton" <neil@gemini-it.com>]

Hi,
	Thanks for all the replies. Error correction was originally disabled due to
issues with new hardware connecting to the existing system, and the fact
that a lot of the old hardware out there, will not connect to the Couriers
unless EC is disabled (it's true !!).

	I appreciate the fact that EC is there to 'stabilise' a connection and to
filter out bad noise, but we have a check digit routine for that (as indeed
do the modems). The problem I have though, it that if it is echo, I would
expect it on most characters not just the odd one. If it was line noise
(which I expect is probably closer to the truth) then the modems must STILL
be providing some sort of correction for them to resend the character again.
If this is the case then I cannot totally disable error correction, which in
turn means I cannot get rid of this problem.

	I've emailed 3com about this, but got a simple response of turn off echo in
hyperterminal, which considering I never mentioned any terminal emulation
software (they got pretty much the same email I sent here) I'm intrigued at
their response.

	I'm going to try later on using the modem in it's default "AT&F1" state and
see what happens. But I know that this will cause problems further down the
line with other modems connecting in the field.

Many thanks for the responses
Regards
Neil Everton

-----Original Message-----
From: rich+ml@lclogic.com [mailto:rich+ml@lclogic.com]
Sent: 03 August 2002 22:31
To: Neil Everton
Cc: Linux-Serial@Vger. Kernel. Org
Subject: Re: [OT(ish] Duplicate Character between modems


Since you don't have an issue when error-correction is on, then I'd say
that pretty much rules out any software-based cause of your extra
character.

What the ec hides is line noise, why do you suppose it was invented to
begin with? Noise on a serial device turns into bogus serial characters,
and you end up with something like thi~@#&!~!#ER$!@#as4 NO CARRIER

On Fri, 2 Aug 2002, Neil Everton wrote:

> Date: Fri, 2 Aug 2002 21:41:09 +0100
> From: Neil Everton <neil@gemini-it.com>
> To: "Linux-Serial@Vger. Kernel. Org" <linux-serial@vger.kernel.org>
> Subject: [OT(ish] Duplicate Character between modems
>
> Hi,
> 	My apologies for sending this here, but I reckon you guys are probably
the
> best bet I have of finding a solution to my problem (esp. as it indirectly
> relates to serial programming)
>
> 	I'm currently porting an application from DOS to Linux (RH7.2 with
Cyclades
> Cyclom-Y (hi henrique!!)).
>
> 	My issue is this, if I send down the modem ;
> "ABCDEFGHI"
> 	the receiving end (very infrequently) receives
> "AABCDEFGHI"
> 	This is caught by the check digit routine and the line is NAK'd ready for
a
> re-transmit. It will usually work the second or third time. This has found
a
> bug in the 3rd party transmitting software, but they're taking the
attitude
> of the existing system works, so you fix your end !!! (the bug if your
> interested is after sending a NAK they drop the connection, instead of
> waiting for 5 retries)
>
> 	The existing DOS system uses old modems such as Pace Linnet (1200 baud)
and
> an even older modem called a Compact which you cannot do anything with, no
> init string, no RING or CONNECT message, just straight into getting the
> data.
>
> 	The new system is using 3Com Courier V.everything modems. I can replicate
> the problem on other modems as well, so it's not a courier issue. We have
to
> disable any error correction, compression (except MNP), and set the flow
> control to hardware. As far as I can see I've disable pretty much
> everything, but I still occasionally get these duplicated characters. It
> used to be quite bad and then I realised that the Courier set the flow
> control on both the transmit and receive stacks seperately. Once I'd set
the
> flow control to hardware for both stacks it made the issue a lot better,
but
> I still get it.
>
> 	I can normally get this issue (at home) at least every 3rd order, luckily
> it's not so bad at the clients end. My home setup incidentally doesn't
have
> a cyclades card, but I've sniffed at the modem connection and I definatly
> don't send the duplicate characters to the modem, they're adding in
between
> the modems.
>
> 	The modem I'm sending on is a generic 56k modem, but we're getting the
> issue on 300 baud modems, with the Courier set to only connect at 300. The
> connect message is simply 'CONNECT' so should be giving no error
correction,
> compression, etc. it should be a standard V21 connection. There might be
an
> issue with fast modems connecting at V21 I'm not aware of, but I'm at a
> loss. My client is also concerned with the issue (understandably) and are
> also in the 'it works on the existing system' mentality.
>
> Any help/pointers/advice gratefully received
> Regards
> Neil Everton
>
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