WARNING: MNP Protocol Will Not Transmit BREAKs & MultiTech Info

[Barry A. Burke]

I have recently replaced all my dial-in modems with MultiTech 224's w/MNP
error correction.  In testing these modems I uncovered a somewhat
significant problem with MNP that may be of interest to all:

    MNP (Level 2 & Level 3) DOES NOT TRANSPORT BREAKS (2 second	spacing) 

This has been confirmed by both MultiTech and Microcom.  Microcom says
"Maybe in a Future release (level 4?)"...don't count on it.

Generally, this is not a problem because UNIX does not depend on the BREAK
signal (note that several other OS's do: VMS, for one).  But, if you want
to use the modem to support 2400/1200/300 access by taking advantage of
getty's BREAK-recognition auto-baud feature, you're stuck.  Unable to send
breaks, one must attempt sending NULLs fast enough to let getty recognize
them- this requires the FASTEST baud rate be the default (so that the NULLs
are long enough to be recognized: a 2400 baud null is 1/2 as long as needed
to be recognized at 1200 baud...most terminals don't auto repeat fast
enough to allow 1200 baud recognition of 2400 baud NULLs).

Of course(?), the MultiTech modems (and others I presume) will in fact pass
a BREAK properly when NOT in RELIABLE (i.e. MNP) mode at 300, 1200, & 2400
Baud.  

I'm opting to set the gettytab to default to 2400, and step through 2400 ->
1200 -> 300 -> 2400 ...  Then users can dial in at 2400 MNP & No MNP, 1200
No MNP (requires 1 BREAK), 1200 MNP (several NULLS & some luck), and 300 No
MNP (requires 2 BREAKs).  Thus we can take advantage of MNP where it's
needed most (2400 baud)!!

A final note for any MultiTech 224EH users out there:  in order to get your
incoming modems to answer in Auto-Reliable mode, you'll need to execute the
following commands to set the modem up:

	AT&E1		(set auto-reliable)
	AT&W		(write current config as default)

The second command is currently undocumented, but required to assure the
modem stays in auto-reliable if/when powered down.  You'll probably also
want to disable Command Responses, Command Echo, and Command Mode in order
to be sure getty doesn't get garbage when the connection is established
(the modem always says CONNECT unless Command Responses are disabled). I
set my switches:

		1 2 3 4 5 6 7 8    1 2 3 4

		U D U D U U U D    U U U D	U=up, D=down

So far, I am very happy with the MultiTech 224's, and I woudl recommend a
look at them by anyone who's considering 2400 baud (or any other speed)
modems.  These are Fully Hayes command compatible (except you'll need
version 2.1 of Smartcom II- 2.0 doesn't understand how to deal with the
2400 baud device & sets it to 300 baud....).
-- 
LIVE:	Barry A. Burke, (617) 965-8480 x26
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