Synchronus Communication

		Synchronus Communication (External modems only)

Synchronous communication mode provides an additional means of establishing a connection between the DTE and the modem. It also provides compatibility with other synchronous modems and with communications ports on a variety of mainframe computers or other multiplex equipment.

&Q1 Synchronous mode (asynchronous command) :

The DTE interface is initially asynchronous to allow dialling to be performed using AT commands. When a successful connection is established the modem goes on-line in synchronous mode. The modem hangs up and returns to asynchronous command mode on detection of carrier loss or an on-to-off transition on DTR. This mode is used to support DTE's that can communicate both synchronously and asynchronously over the same RS232 port.

&Q2 Synchronous mode (DTR number dialling) :

This mode is mainly intended for a synchronous only terminal, although an asynchronous DTE is required to program the number to be dialled in number store 0. Upon detection of an on-to-off transition on DTR, the modem goes off hook and dials the number stored in location 0. Once connected, loss of carrier or an on-to-off transition on DTR makes the modem hang up and return to asynchronous command mode.

&Q3 Synchronous mode (manual number dialling) :

This mode is normally used to support a dedicated synchronous data terminal. The call must be manually established on a series connected telephone while DTR is off. The handshaking proceeds when DTR becomes active; at which time the phone should be hung up. The modem terminates the call when the carrier is lost or an on-to-off transition is detected on DTR.

\N Asynchronous modes :

De-selection of synchronous mode is best achieved by using the appropriate asynchronous error correction operating mode command. Issuing a \N command automatically forces the &Q setting to the appropriate non-synchronous value.

&X Synchronous clock source :

The &X command selects the source of the synchronous transmit clock. This can be internally generated by the modem (&X0), externally generated from the DTE (&X1) or derived from the receive signal (&X2). The synchronous receive clock is always derived from the received carrier.

The choice of which transmit clock source to use may be influenced by the way in which the remote modem is configured. The rule is that there must be one, and only one, clock source for each data transfer direction. This clock may be generated by the modem if the DTE to which it is connected does not supply a clock. If the modem does generate the clock, care should be taken to ensure that both modems do not derive their transmit clocks from their respective receive signal (&X2).