T h e bus is organize d in to th ree sets o f signal lines:
— D a ta b u s, 8 signal lines
— D ata b y te tra n sfe r co ntrol bus, 3 signal lines
— G eneral in terfac e m anagem ent bus, 5 signal lines
D ata Bus
D I0 1 - D I 0 8
D ata In p u t O u tp u t message bytes are c a rried o n th e D IO signal lines in a
b it- parallel b y te serial fo rm , s ynch rono usly, and ge n e rally in a b id irectio na l
m anner.
D a ta B yte T ra nsfe r C o n tr o l Bus
D A V
D a ta V a lid is used to in dic a te th e a v a ila b ility an d v a lid ity o f in fo rm atio n
on th e D IO signal lines.
N R F D
N o t Ready F o r Da ta is used to in dicate th e co nd itio n o f readyness o f devices
to ac ce p t da ta .
N D A C
N o t D ata A c c epte d is used to indicate th e co nd itio n o f accep tance o f d ata
b y devices.
T h e D A V N R F D and N D A C signal lines o p era te in a three w ire (in te rlo ck ed ) handshake
process t o tran sfe r each d ata b y te across th e in terfa ce.
Gen eral In te rfa c e M a nag e m ent Bus
A T N
A tte n tio n is used o y a c o n tro lle r to sp ec ify ho w da ta on th e D IO signal
lines is to be interpre te d and w h ic h devices must respond to th e da ta.
IF C
Interfa ce C lea r is used b y a co n tro lle r to place th e in te rfa ce system
(p ortio ns o f w h ich are c ontaine d in all in te rco n n e cted devices) in a kn ow n
quiescent state.
S R Q Service R eq uest is used b y a device to in dica te the need fo r a tte n tio n , and
to request an in te rru p tio n o f th e c u rre n t sequence o f events.
R E N
R em o te Enab le is used by a c o n tro lle r in co nju n c tio n w ith o th e r messages
to select b e tw ee n tw o a ltern a te sources o f device pro g ram m ing d ata .
E O I
End O r Id e n t ify is used b y th e ta lk e r to indica te the e n d o f a m u ltip le b y te
tra ns fer sequence o r, in co nju n c tio n w ith A T N to ex e cu te a polling sequence.
IEEE - 488 Bus Signals
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