PDP11 PeripheralsHbk 1972 - Trailing-Edge

A Detect Answer option is used. The 801 retains line control and looks for
an answering tone, from the called station. Upon receiving the tone the
modem is connected to the line, Data Set Status is asserted and a program
interrupt is generated (DSS = 1). This stops the Abandon Call and Retry
timer which would have been initiated had no tone been received. These, in
turn, would have generated a signal to the DN 11 and cause a program interrupt
with the Abandon Call and Retry bit set (ACR = 1). The program
would then either retry or drop the call.
b) If using modems without the automat!c handshaking feature:
The End-of-Number (EON) mode must be used. EON is sent after the last
digit has been dialed. This causes the 801 to connect the modem to the
line and assert Data Set Status (DSS = 1). However, the modem and its
controller must be able to determine when the called station has answered
and is sending data. To do this, it is necessary to use an 801 with option
"Y"(available from the Telephone Company). This option lets the Abandon
Call and Retry timer continue running even after the DSS bit has been set.
When the ACR timer times out it will notify the user of the line to check if
data is being received by the modem
14. There are two options available when terminating a call:
a) The Call Request bit is set to zero (FCRQ = 0). This will remain until the
Data Line Occupied bit also goes to zero (DLO=O), which is a necessary condition
before a new call can be initiated.
b) If the 801 option "Z"is used, the call can be terminated by clearing Data
Terminal Ready in the modem. In this case, dropping Call Request will not
terminate the call. However, it must be dropped before a new call can be
attempted.
15. Should the 801 lose power during a call an interrupt will be generated and the
Power Off bit will be set (PWI = 1). The interface will not return an interrupt if
the Call Request bit is set with the power off (FCRQ = 1).
8.3.3 Programming
Each ACU interface contains one register and therefore requires one l6-bit address.
Address space has been assigned for 64 interfaces. The four addresses for
the four interfaces that can be plugged into one system unit must be consecutive
addresses starting with 175XXO where XX = 20 for the first line. If only one line is
in use, it uses address 175200. Interface number 2 has address 175202, and interface
number 64 has address 175376.
Note: In addition to the individual Interrupt Enable bit for each interface, there is
a master enable bit associated with line number 1 of a given system unit. It enables
the interrupts for the entire group. The master enable bit on lines 2 through
4 of a given system unit are ignored by the interface.
Each set of four DNll's requires one interrupt vector. The vector address for
communications options are assigned in the range from 300 to 777 according to
the following convention:
1. The first DC1l assigned in a system will be given vector address 300, the next
one (if any) will be given vector 310 etc.
2. Then KUl's will be assigned in order.
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