PMDF System Manager's Guide
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37.5.3 Configuring Service Class 2 Modems

If you intend to use FAX receive, then take the steps outlined in Section 37.5.1 to ensure that the SYSGEN parameter MAXBUF exceeds 2048 bytes.

PMDF-FAX is designed to drive ZyXEL U1496, ZyXEL Omni, ZyXEL Elite, and MultiTech 1432 series FAX modems without requiring any configuration changes to the modems themselves. MultiTech ZDX series modems are also supported, but do require a configuration change from the factory default settings; see Section 37.5.3.4 for details.

However, note that the factory default settings may not be appropriate for you:
  Default Flow  
Modem Control Method Comments
MultiTech 1432 series CTS/RTS  
MultiTech ZDX series CTS/RTS  
ZyXEL Elite series CTS/RTS  
ZyXEL Omni series CTS/RTS  
ZyXEL U1496 series CTS/RTS Requires 19200 baud or faster
A nice feature of the MultiTech 1432 series modems is that they list the factory defaults on their underside and have switches on their side with which you may physically change the default settings. See Sections 37.5.3.1 and 37.5.3.2 for further details on flow control methods and modem speed.

When wiring your modem, use a "straight" 25 wire RS232 cable if possible. (Straight here means that pin N on the DTE end is wired to pin N on the DCE end, N = 1, 2, 3, ..., 25.) If you have fewer than 25 wires, then the important RS232 signals to wire are 2, 3, 4, 5, 6, 7, 8, and 20 (TXD, RXD, RTS, CTS, DSR, GND, CD, and DTR).

Many of the terminal port characteristics have no effect on the operation of PMDF-FAX: all I/O operations are done via $QIO calls in PASTHRU mode (IO$M_NOFORMAT). Critical settings are made on a temporary basis (IO$_SETMODE) by the G3_TO_FAX channel when conversing with the FAX modem: TTSYNC on, TYPEAHEAD on. However, the terminal line should, at a minimum, be set as follows 


$ SET TERMINAL/PERMANENT/ALTYPEAHD/NOTYPE_AHEAD/NOHANGUP/EIGHTBIT device-name
A suggested set of terminal characteristic settings are shown in Example 37-11.

If the modem is connected to a terminal server, then the terminal server port should be set REMOTE, with the only enabled characteristics being Input Flow Control and Output Flow Control. When the LT device associated with the port is created, be sure to create it as a non-queued device with the /NOQUEUE qualifier. For convenience, a sample display of appropriate port settings are show in Example 37-12.

37.5.3.1 Flow Control

Most Service Class 2 modems require that CTS/RTS flow control be used for FAX operations. If this is the case with your modem, then be sure to use a terminal port which supports CTS/RTS flow control.

If your modem allows a choice of flow control methods for Service Class 2 FAX operations, then configure the terminal port and modem as you see fit. Save the desired modem setting into the modem's default profile; i.e., the profile which is loaded when a simple modem reset command is issued. If you'd prefer to have PMDF-FAX set the modem's flow control method each time it uses the modem, make use of the FLOW_CONTROL_CMD option in the G3_TO_FAX and FAX_TO_DATA channel option files.

37.5.3.2 Communication Speed

Many Service Class 2 modems require that a speed of 19200 baud or faster be used between the host computer and modem. If your modem requires 19200 baud, be sure to use a terminal port rated for 19200 baud. VAXstation 3100 ports are not rated for 19200 baud and typically run a bit faster than 19200 baud.

If your modem allows a choice of speeds, then you are strongly encouraged to use 19200 baud or faster. If you elect to use a speed slower than 19200 baud, then you will be unable to send or receive FAXes at 9600 bps.a

37.5.3.3 Changing Modem Settings

Should you wish to alter settings in your modem, you will need to either (1) connect to the modem, effect the changes, and then save the changes into the modem's default profile, or (2) through the use of G3_TO_FAX and FAX_TO_DATA channel option files, have PMDF-FAX make the desired settings each time it uses the modem. Details on (1) are provided below; for (2), see the description of the EXTRA_CMDS and FLOW_CONTROL_CMD options in Section 37.2.17.4.

To connect to the modem in order to alter modem settings, use the SET HOST/DTE command. In the following example, an AT command is first issued in order to see if the modem is responding. Next, the command AT&H4&W is issued which, for a ZyXEL modem, instructs the modem to use XON/XOFF flow control and to save the setting in the default profile. 


$ ALLOCATE TTA4:
%DCL-I-ALLOC, _EXAMPLE$TTA4: allocated 
$ SET TERM/TYPE_AHEAD TTA4:
$ SET HOST/DTE TTA4:
%REM-I-TOQUIT, connection established 
Press Ctrl/\ to quit, Ctrl/@ for command mode 
AT[RETURN]
OK 
AT&H4&W[RETURN]
OK 
^\
%REM-S-END, control returned to node EXAMPLE:: 
$ DEALLOCATE TTA4:
$

37.5.3.4 Configuring MultiTech ZDX Series Modems

For MultiTech ZDX series modems you should configure the modem with the command 


AT&SF1&S0&W0
so as to force the modem to always assert DSR rather than have it follow DCD. Failure to do that may lead to DSR unexpectedly dropping as DCD can change state repeatedly during the course of a normal FAX transmission. This will, in turn, cause a terminal server to either hangup the port or cease transmitting to the modem while DSR is low.

This command need only be issued once to the modem; the &W0 portion of the command will commit the setting to non-volatile memory in the modem.

See Section 37.5.3.3 for information on transmitting this command to the modem from your system.

37.5.3.5 Changing Modem Commands

Many of the modem commands which PMDF-FAX must use are not covered by the Service Class 2 modem specification and are manufacturer specific. PMDF-FAX determines which commands to use by determining the modem's manufacturer with the Service Class 2 modem command AT+FMFR?. However, with older versions of some modems, it may be necessary to instruct PMDF-FAX to use different commands; likewise if you attempt to use an unsupported Service Class 2 modem. Table 37-5 summarizes the G3_TO_FAX and FAX_TO_DATA channel options which may be used to specify the commands which PMDF-FAX should use for various operations. Those options assume that the command is prefixed with "AT". Thus, to specify that the command AT&K4 should be used to set the flow control method, the option file should contain the entry 


FLOW_CONTROL_CMD=&K4

Table 37-5 Controlling Modem Commands
Option   Usage
DC2_CMD <- Ordinal value of the character to transmit to initiate reception of page data; normally 34 (CTRL/R) however some modems may require 33 (CTRL/Q).
ECHO_OFF_CMD <- Disable command echo; normally E0.
FLOW_CONTROL_CMD <-> Desired flow control method; varies with manufacturer.
MANUFAC_QUERY_CMD <-> Query modem manufacturer; normally +FMFR?.
NO_ANSWER_CMD <-> Disable auto answer; normally S0=0.
ON_HOOK_CMD <-> Hang-up the phone line; normally H0.
RESULTS_ON_CMD <-> Return response codes; normally Q0.
SOFT_RESET_CMD <-> Reset the modem to the desired profile; normally Z or Z0.
VERBOSE_OFF_CMD -> Return numeric response codes; normally V0.
VERBOSE_ON_CMD <-> Return verbose response codes; normally V1.

Notation
See Sections 37.2.17.4 and 37.3.6.1 for complete descriptions of the options shown in Table 37-5.

Note

a Whereas the host-to-modem protocol is asynchronous and requires transmitting 10 bits for each 8 bits of data, the communication protocol between FAX devices is synchronous and transmits 8 bits for each 8 bits of data. Thus, for the host system to keep up with received data from the modem or to feed data to the modem sufficiently fast, the host-to-modem data rate must be approximately 25% faster than the FAX-to-FAX rate. Thus, to send or receive FAXes at 9600 bps, the host system must communicate with the modem at a speed in excess of 12000 baud.

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