Control Data Cyber 170 State Hardware Reference Manual page 214

PP Programming
Input/Output Transfers
The following paragraphs d i s c u s s input/output t r a n s f e r s with t h e PP.
Data Input Sequence
The e x t e r n a l device sends d a t a ( f i g u r e
5-8)
t o the PP v i a t h e c o n t r o l l e r a s
follows :
The PP p l a c e s
a
function word i n t h e channel r e g i s t e r and s e t s t h e f u l l
f l a g and t h e channel a c t i v e f l a g . A t t h e same time, t h e PP sends t h e f i r s t
of a group of words and f u n c t i o n s i g n a l s t o a l l c o n t r o l l e r s . The f u n c t i o n
s i g n a l s cause a l l c o n t r o l l e r s t o sample the words and i d e n t i f y t h e words a s
f u n c t i o n codes r a t h e r than d a t a words. Connect codes s e l e c t c o n t r o l l e r s
and modes of operation and c l e a r nonselected c o n t r o l l e r s . Only s e l e c t e d
c o n t r o l l e r s a r e connected.
The c o n t r o l l e r sends an i n a c t i v e s i g n a l t o the PP, i n d i c a t i n g acceptance of
t h e f u n c t i o n code. The s i g n a l drops t h e channel a c t i v e f l a g , which i n
t u r n , drops t h e f u l l f l a g and c l e a r s t h e channel r e g i s t e r .
The PP s e t s t h e channel a c t i v e f l a g and sends a n a c t i v e s i g n a l t o the
c o n t r o l l e r , which s i g n a l s the i n p u t equipment t o s t a r t sending d a t a .
The i n p u t equipment reads
a
12-bit d a t a word p l u s 1 p a r i t y b i t and then
sends the word with p a r i t y t o t h e channel r e g i s t e r with a f u l l s i g n a l ,
which s e t s t h e channel f u l l f l a g (10 t o 1 5 nanoseconds a f t e r the data
a r r i v e s ) .
The PP s t o r e s the word, drops t h e f u l l f l a g , and r e t u r n s
an
empty s i g n a l ,
i n d i c a t i n g acceptance of t h e word.
The i n p u t equipment c l e a r s i t s d a t a
r e g i s t e r and prepares t o send t h e next word.
Steps 4 and 5 repeat f o r each word t r a n s f e r r e d .
A t
t h e end of the t r a n s f e r , t h e c o n t r o l l e r c l e a r s i t s a c t i v e condition and
sends an i n a c t i v e s i g n a l t o t h e PP t o i n d i c a t e t h e end of t h e d a t a .
The
s i g n a l c l e a r s the channel a c t i v e f l a g t o disconnect t h e c o n t r o l l e r and the
PP from t h e channel.
As an a l t e r n a t i v e , t h e PP may choose t o d i s c o ~ e c t from t h e channel before
t h e i n p u t equipment has s e n t a l l i t s d a t a .
The PP does t h i s by dropping
t h e a c t i v e f l a g and sending an i n a c t i v e s i g n a l t o t h e c o n t r o l l e r , which
immediately c l e a r s i t s a c t i v e c o n d i t i o n and sends no more d a t a , although
the input equipment may continue t o the end of i t s record o r cycle ( f o r
example, a magnetic tape u n i t would continue t o end-of-record and s t o p i n
the record gap).