Article 5042 of vmsnet.networks.tcp-ip.cmu-tek:
On Fri, 28 Jun 1996, Stan Gierlotka wrote:

> Dear all,
> I've been trying to set up a dial-up SLIP line according
> to the FAQ i.e. my "at-work" VAX serves my "at-home" PC.
> The connection seems to get established OK, but the 
> packets don't get through. Actually watching the modem 
> LED's I deduced that the VAX-end modem don't send 
> anything to VAX. Here are the symptoms:
> - when pinging from VAX to PC the PC responds (both RD 
> and TD LED's at the PC-modem flash) but VAX gets nothing 
> or does not understand the response.
> - when pinging from PC to VAX only RD LED at the 
> VAX-modem flashes and there is no responce send back to 
> the PC.
> 
> I am a bit suspicious about hardware-flow-control issue. 
> It is (and must be) enabled at the PC-end, but what 
> about VAX? I tried a /commsync swith when setting
> my tta2 characteristics but then SLPDRV could not talk 
> to the port at all. I set up the modem to ignore RTS 
> signal but nothing changed. Xon/Xoff is disabled of 
> course so how is the flow control between modem and VAX 
> realized? Could someone put some light on it?
> What else can I do? What should I look at?
> TIA
> Stan
> system@vax.unipress.waw.pl
> 

There are a *lot* of things that could be causing you problems, but I'll
start with a few suggestions and you can repost your help-request if these
don't help.

One common problem is that the VAX follows the Electronics Industries
Association (EIA) standards for RS232 connections (to the modem, in this
case) and PC's don't.  Specifically, the PC ignores Data Set Ready (DSR)
signals (pin 6 on your 25-conductor cable).  Modem manufacturers usually
configure their modems for the defacto PC standard rather than the
officially promolgated EIA standard so you need to issue a setup command to
your VAX's modem to tell it to provide a DSR signal (AT&S1 does it on my
modem). 

You'll need some form of flow control, and many will work, but extensive
testing has shown me that setting hardware flow control on both ends of the
connection provides the fastest transfer speeds with high speed modems. 
This implies setting up the terminal port as well as the modem.  The newer
modems provide error correction and data compression on the fly.  Note that
previoNote that not all terminal ports support hardware flow control (RTS/CTS),
notably ones with the MMJ connections instead of RS232.  You'll have to use
XON/XOFF in that case.

So, for a USRobotics 28.8kbs modem connected to a VAX, I set it up with the
command AT&K3&S1.  Assuming the modem is connected to terminal port TXA1, I
would issue a SET TERMINAL TXA1 /PERM/EIGHTBIT/FULLDUP/MODEM/NOHOST/NOTT and
a whole lot more /etc. to get my terminal set up like this:

Terminal: _TXA7:      Device_Type: Unknown       Owner: _TWA26:
                                              Username: RPULLER

   Input:  19200      LFfill:  0      Width: 511      Parity: None
   Output: 19200      CRfill:  0      Page:   24

Terminal Characteristics:
   Passall            No Echo            Type_ahead         No Escape
   No Hostsync        No TTsync          Lowercase          No Tab
   Wrap               Scope              Remote             Eightbit
   No Broadcast       No Readsync        No Form            Fulldup
   Modem              No Local_echo      No Autobaud        Hangup
   No Brdcstmbx       DMA                Altypeahd          Set_speed
   Line Editing       Insert editing     No Fallback        Dialup
   No Secure server   Disconnect         Pasthru            No Syspassword
   No SIXEL Graphics  No Soft Characters No Printer Port    Numeric Keypad
   No ANSI_CRT        No Regis           No Block_mode      NoAdvanced_video
   No Edit_mode       No DEC_CRT         No DEC_CRT2        No DEC_CRT3
   No DEC_CRT4

Setting the Altypeahd characteristic provides an alternate buffer for I/O,
but it doesn't take effect until your next login.  You need to set that
buffer size in SYSGEN to something like TTY_ALTYPAHD = 2000 to get the
benefit of a larger buffer (I use 17000 to provide for large sliding windows
with KERMIT and it works well with FTP).

I Don't normally deal with PC's but I have noted that most modems have been set (probably by ill-behaved communications software) to
ignore the Carrier Detect (CD) signal (pin 8).  You may need to set your
PC's modem to follow CD (AT&C1).

|  Rob Puller       |  Robley S. Puller, P.E.   Consulting Engineer    |
|  rpuller@afn.org  |  2130 N.W. 51st Terrace   Gainesville, FL 32605  |