Am79C901A
35
P R E L I M I N A R Y
Data Transmit Timing
A data symbol interval begins with the start of transmis-
sion of a pulse as shown in Figure 19. Transmit Symbol
timing (in TICs) is measured from this point (TIC = 0).
Depending on the data code, the next pulse may begin
at any PULSE_POSITION_N where N = 0 to 24. Each
position is separated from the previous one by one TIC.
PULSE_POSITION_0 occurs at a value defined in
Table 8 which determines the transmission speed.
When a pulse begins transmission, the previous sym-
bol interval ends and a new one begins immediately.
Table 8.
Blanking Interval Speed Settings
Figure 19.
Transmit Data Symbol Timing
Data Receive Timing
The incoming waveform is formed from the transmitted
pulse along with any distortions and reflections that
occur in the wiring network. The receiver detects the
point at which the envelope of the received waveform
crosses a set threshold. See Figure 20.
Immediately after the threshold crossing, the receiver
disables any further detection for a period ISBI-3 TICs
(HPR28, ISBI_SLOW or ISBI_FAST) starting with the
detection of the pulse peak.
The receiver is then re-enabled for pulse detection.
Upon reception of the next pulse, the receiver mea-
sures the elapsed time from the previous pulse.
This value is then placed in the nearest pulse posi-
tion bin (one of 25) where pulse position 0 is at
PULSE_POSITION_0 and each subsequent posi-
tion is spaced one TIC from the previous one as de-
fined in the
Data Transmit Timing
section. Data
symbol intervals are therefore variable and depend
on the encoded data.
Figure 20.
Receive Symbol Timing
Speed Setting
LOW_SPEED
Nominal Data
Rate
0.7 Mbps
PULSE_POSITION_0
Value
(in TICs)
44
HIGH_SPEED
1.0 Mbps
28
Symbol 1
Symbol 2
Pulse 0
Data Blanking interval (DISBI)
Pulse 1
Pulse 2
1 TIC
START_TX_PULSE
TIC=0
END_TX_PULSE
time
PULSE_POSITION_0
time
Position 1
Position n1
n=0-24
Position 0
Position 1
Position n2
Transmitter
22304B-21
Symbol 1
Symbol 2
Pulse 0
Detected Envelope
Pulse 1
Pulse 2
egin of receive
anking interval
END_DATA_BLANK
Position 1
Position n1
Position 0
Position 1
Position n2
Position 0
Data slice
threshold
Receiver
22304B-22