Automatic Calling Unit Interface, Dnll - Digital Equipment pdp11 Handbook

8.3
Automatic Calling Unit Interface ON 11
8 .. 3.1
Introduction
With the DN 11 and a Bel! 801 Automatic Calling Unit (ACU), any PDP-11 can dial
any telephone number in the Direct Distance Dial Network and establish a data
link. The DN 11 is a digit-buffered interface, and digits to be dialed are presented
as four-bit binary numbers. The interface drives the ACU with EIA-232-C voltages
and is connected via a standard 25-pin plug.
The programmer has access to all lines of the 801 through the DN 1l. The 801
presents the following leads to the DN 11: Power Oft and Indicator, Data Line Oc-
cupied, Abandon Call and Retry, Data Set Status and Present Next Digit. The
DNll provides the following leads to the 801: Digit Present, Call Request and
four Digit Leads.
Because the PDP-ll UNIBUS serves as a multiplexer, multiple automatic calling
units can be added to the PDP-11. One PDP-ll System Unit accepts up to four
801 ACU Interfaces. Each interface looks like one device to the UNIBUS.
8.3.2
The Sequence of Operation
The following describes the use of the DN 11 to originate a ODD call. This is an
automated version of the procedure that everyone goes through when placing a
telephone call.
l. Check for 801 power on (PWI
=
0).
2. Check for unoccupied data line (DLO
=
0).
3. Set Call Request bit (FCRQ
=
1).
4. The 801 will seize the line on receiving the dial tone and assert Present Next
Digit which causes a PDP-11 program interrupt (FPND
=
1).
5. The line is now in use and the Data Line Occupied bit is set (DLO
=
1).
6. The first digit to be dialed is provided by loading the four least significant bits
of the byte into the digit bits (8 to 11) of the DN 11 status register. The upper
four bits of the byte are read-only and can have any value during the loading
of the four low-order bits.
7. The 801 is informed that the 1st digit has been loaded by asserting the Digit
Present Bit (FDPR
=
1).
8. The 801 then reads Digit leads 1 through 4 and lowers Present Next Digit Lead
(FPND =0).
9. The hardware responds and lowers Digit Present Lead (FDPR
=
0).
10. The 801 then dials the first digit and again raises Present Next Digit Lead
(FPND
=
1).
1l. The next digit is loaded and the Digit Present bit is asserted (FDPR
~
1).
12.Sequences 6 through 10 are repeated until all digits have been dialed.
13. When the last digit has been dialed, one of two procedures must be used to
complete the call.
a) If "handshaking signals"are used (Bel! 100 series modems or equiva·
lent):
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