Mode Selection

The operating mode, either V.25bis or normal AT command mode, is determined at power-on by the DIP switch settings. The two V.25bis switches have four possible settings:

Switch 3     Switch 4      Meaning                                                            

                                                                                              

up*          up*           At power-on: AT command operation.                                 

                           On soft reset: S94 determines mode                                 

                                                                                              

up           down          Always async V.25bis                                               

                                                                                              

down         up            Always sync V.25bis (HDLC framing)                                 

                                                                                              

down         down          Always sync V.25bis (Character framing)

* factory default

Note: This chart views the modem from the front, top side up. Switches are numbered from left to right as 1,2,3,4.

Sometimes a reset is necessary: V.25bis is more than a command language; it is a complete specification of modem behavior (auto answering, RS232 signals, etc.) A modem cannot simply accept "V.25bis commands"sometimes and "AT commands"at other times; which set of rules would it use to determine correct modem behavior? The only way to switch cleanly between the two specifications is to require a total modem reset.

If the switches are set in any of the three V.25bis positions, the command language and overall behavior are as specified in the V.25bis recommendation. The modem will not accept AT commands when the DIP switches are set for V.25bis. Switch 1, the "dumb strap"DIP switch, determines whether "addressed call"or "direct call"procedures are used: dumb implies direct, not dumb implies addressed.

If switches 3 and 4 are both UP (factory setting), the operating mode at power-on is normal AT command mode. On a soft reset (Z command), however, the operating mode is determined by the stored value of S94 (the three least-significant bits). The three least significant bits of S94 apply when the DIP switches are in the UP UP position.

If DIP switches 3 and 4 are both UP UP and the modem is not in Dumb Mode (factory setting), the operating mode at power-on is normal AT command mode rather than V25bis. On a soft reset (Zn command), however, the operating mode is determined by the stored value of S94 (the three least-significant bits).

Note: The three least significant bits of S94 apply when the DIP switches are in the UP UP position.

The following chart lists the values for S94:

Value       Meaning                                   

0           Standard AT command operation             

1           Async V.25bis                             

2           Sync V.25bis (HDLC framing)               

3           Sync V.25bis (BSC framing)                

add 4       direct calling procedures                 

add 8       EBCDIC instead of ASCII                   

add 16      NRZI instead of NRZ

Note: Values correspond to the switch settings if [switch 1] [switch 2] [switch 3] [switch 4] are considered as a three-digit binary number and assign UP=0, DOWN=1.

Values 1-3 use addressed calling procedures (the modem is operated via commands) whereas values 5-7 use direct calling procedures (the modem is operated purely through RS232 control signals.) When switches 3 and 4 are UP UP, the calling procedures are determined by S94 rather than the "dumb strap" DIP switch.

The only way to switch modes is to reset the modem (by cycling power, issuing an ATZ command.)

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