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@(#)dn.4 4.1 (Berkeley) 05/15/85
A dn-11 interface can control as many as four call units. In normal operation a data link is established by opening the file associated with a call unit, sending the desired phone number in a single write (2) operation, then opening the terminal line attached to the appropriate outgoing modem. If the phone call was successful and the remote host presented a carrier signal, the open on the terminal line should be successful. At this point the call unit should be closed and a link has been established. This sequence is normally performed for the user by one of cu (1), uucp (1), or dnd (8).
Each controller/call unit pair has different programming characteristics which impact the programmer; the dn-11 interface presents a "raw interface" rather than imbed local dependencies in the device driver. For example, the Racal/Vadic 811 call unit is multiplexed, providing the user with the ability to place phones calls on as many as 64 separate phone lines from a single interface. This multiplexing is carried out based on the first two digits of the phone number presented by the user (the first digit selects the modem's rack and modem type, while the second digit selects the specific modem in the rack).
The legal dn-11 dial codes are: 0-9, * (: is a synonym), # (; is a synonym), = (w is a synonym), and -. A phone number may optionally be terminated with a < or e. Phone numbers may be no more than 40 characters long.
By convention /dev/cu[al]0 are connected to a 300 baud interface, while /dev/cu[al]1 provides a 1200 baud interface.
/dev/cul[0-9] associated outgoing terminal line