Handshake Lines; Handshake Timing Diagram - Watkins-Johnson Company WJ-8718A/MFP Instruction Supplement

WJ-8718A/MFP FIGURE 2-7
2.4.5.3.2 Handshake Lines
A three-wire handshake sequence facilitates the asynchr-onous, bidirectional trans-
fer of each byte of data for up to 14 receivers and one controller. The data transfer rate
adjusts to the slowest active device on the bus, allowing units with differ-ent input/output
speeds to be interconnected. The handshake lines are DAV (Data Valid), NRFD (Not Ready For
Data), and NDAC (Not Data Accepted). When reading the following description of each signal,
refer to Figure 2-7, Handshake Timing Diagram.
DAV - The level on the DAV line is established by the transmitting
device to indicate the availability and validity of information on the
DIO data lines. When DAV is high, the data is considered invalid;
when the signal level is low, the data is considered valid.
NRFD - The logic level of the NRFD signal indicates the condition of
readiness of device(s) connected to the bus. When one or more
devices are not ready for data, the NRFD line is logic low (true).
However; when all devices on the bus are ready to accept data, the
signal goes high (false: all are ready to acc·ept data).
NDAC - The NDAC handshake signal indicates the condition of a
device(s) to accept data. This line is logic low (true: data has not
been accepted) until data are accepted by the device(s) on the bus.
When the last device has accepted the data, the NDAC signal goes
high.
DATA
__
---'x~
___' x _
nTTl
ALL
...R_£_A_D_Y _
SOME
READY
DAV
NRFD
NDAC
--'
I
I
ALL READY
I
I
I
I
I
DATA TRANSFER
BEGINS
VALID
NONE READY
1----_ _-
I
I
I
I
I
I
r:ITWALL
ACCEPT
I
ACCEPT
I
TRANSFER
ENDS
Figure 2-7. Handshake Timing Diagram
2-25