Re: Need Point-to-Point Patch for 14000 BAUD rate

[dempson@actrix.gen.nz (David Empson)]

stephen e buggie <buggie@indus.unm.edu> wrote:

> I use POINT-TO-POINT modem software.  I've now got a Promodem 144e modem
> by Prometheus.  Its an external modem connected to the modem serial port
> of the Rom 3 IIgs.
> 
> The problem is that the two fastest speeds on Point-to-Point are:
> 
> 9600 BAUD and 19200 BAUD.
> 
> The modem goes 14400 baud at its fastest rate.

You need to distinguish between two speeds here:

- The speed at which the modem transfers data over the telephone line
(the carrier speed, if you like).

- The speed at which the modem and computer exchange data.

These two are not necessarily the same for any modem faster than 2400
bps (and even for some 2400 bps modems).

If the modem supports error correction and/or data compression, then the
computer-to-modem speed must be HIGHER than the modem's carrier speed.

If the modem does not support error correction or data compression, and
either the modem or computer has no support for handshaking, then the
modem's carrier speed will match the speed at which the computer is
connected to the modem.  (This applies to all 300 bps modems, all 1200
bps modems that I know of, and most 2400 bps modems.)

In the case of a 14k4 modem, you would normally be using error
correction and data compression (either MNP-4 and MNP-5, or V.42 and
V.42bis), so the computer should be set to talk to the modem at a speed
higher than 14400 bps.  I have found that 19200 is sufficient.  (It is
not optimal if you are sending or receiving uncompressed text data, for
example, but it is good enough for most situations.)

The main catch: handshaking is required between the computer and modem,
so that the modem can tell the computer to stop sending data if it
cannot keep up (depending on error recovery, how compressable the data
is, etc.)  The best option is to use hardware handshaking.

This raises the question: does Point-to-Point support hardware
handshaking?  I don't know the answer to this.  If it obeys the IIgs
Control Panel settings, then all you should need to do is enable DTR/DSR
handshaking in the IIgs control panel, and ensure that your modem cable
connects to the modem as follows:


The pins on the IIgs connector are numbered as follows (looking at the
back of the computer; swap these horizontally if you are looking at the
pins on the cable):

  8 7 6
 5   4 3
   2  1

The cable pinout should be:

IIgs    Modem (DB-25)
  1      4             Handshake, computer to modem
  2      5             Handshake, modem to computer
  3      2             Data, computer to modem
  4      7             Signal ground
  5      3             Data, modem to computer
  6     (no connection)
  7      8             Carrier detect, modem to computer
  8     (must be connected to IIgs pin 4 or modem pin 7)


This is a standard "hardware handshake" cable for the IIgs (or Mac).
The traditional modem cable connects the flow control signals (pins 1
and 2) to the DSR and DTR pins on the modem, which are not used for
handshaking.  If you are using an existing cable that was previously
used with a slower modem, you will probably need to get it modified, or
buy a new cable.

In the meantime, your existing cable will only work reliably if you
disable hardware handshake, error correction and data compression
capabilities in the modem, and select 9600 bps on the computer.

> BTW, I have a second identical ProModem 144e that does not work.  The CD
> (carrier detect) lamp is always on.   I looked inside the modem case and
> there are no obviously burned components.  Any suggestions????

This might be a setting: the AT&C command is used with most modems to
specify whether the Carrier Detect signal is to be used.  AT&C0 forces
it to be permanently active, AT&C1 allows it to follow the real carrier
detect state.

If using AT&C1 does fix it, use AT&W to write the updated setting into
the modem's nonvolatile memory.

Ideally, you should do an AT&F to restore factory default settings, then
go through the manual and identify which settings you wish to change,
then write the settings using AT&W.

A subsequent power-on or reset command (ATZ) will restore the saved
settings.

-- 
David Empson
dempson@actrix.gen.nz
Snail mail: P.O. Box 27-103, Wellington, New Zealand