Commodore 2001-8 User Manual page 89

Figure 7-22 shows the relative timing of transfer bus signals during a typical handshake; the bracketed
numbers in the following sequence refer to the changes in signal logic levels in the Figure:
1) NRFD goes high (false) indicating that all listeners are ready for the
next byte of data.
2) The talker puts the next data byte on the data bus and allows the data
signals to settle. This could happen before, after or during (1).
3) The talker tests NFRD, when it is found to be too high, the talker makes
DAV low (true)to inform listeners that the bus data is now valid.
4) As soon as a single listener detects that DAV is low, that listener sets
NRFD low; data is now accepted by all the individual listeners at their own
rate, each of whom release NDAC as they accept the data.
5) NDAC goes high (false) when the slowest of the listeners have accepted
the data.
6) The talker sets DAV high (false) indicating that the bus signals are now
invalid.
7) The listeners note that DAV has gone high and sets NDAC
low (true) completing the handshake. When each listener has processed
the data, they release NFRD. This terminates the sequence for the first
data transfer. The sequence will repeat again, beginning at (a), until all
required data transfers have been completed.
PET/IEEE Bus Timing Constraints
The following limitations should be noted in order to avoid a loss of data:
a) When PET is a listener, it expects DAV to go low within 64 milliseconds
after it has set NFRD high.
b) When PET is a talker, it expects NDAC to go high within 64 milliseconds
after it has set NRFD high.
If these limitations are exceeded, the PET ceases to transfer and sets the appropriate status word (ST).
See Table 7-24.
THE MANAGEMENT BUS
This group of five signal lines controls the state of the data bus and defines its signals; these can be
concerned with data, addresses, or control information (device commands).
The five management signals are:
a) ATN Attention
b) EOI End or
identify
c) IFe Interface
clear
d) SRO Service
request
e) REN Remote
I
enable
Assigns devices to act as listeners
or talkers.
Indicates that the last data byte is
being transferred.
Initializes the data bus. Talkers and
listeners set idle. Same signal as
reset in the PET.
Device tells controller that service is
required. Not implemented in BASIC
but available in PET.
Permanently tied to ground in the
PET.
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