AM79C978VCSLASHW datasheet(79/261 Pages) AMD

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Am79C978

Data Symbols

Data symbols encode data for a much higher transmis-

sion rate, and they do not allow collision detection.

Data Transmit Timing

A data symbol interval begins with the beginning of

transmission of a pulse as shown in Figure 40. Trans-

mit 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 13 which determines the transmission speed.

When a pulse begins transmission, the previous sym-

bol interval ends and a new one begins immediately.

Table 13.

Blanking Interval Speed Settings

Figure 40.

Transmit Data Symbol Timing

Data Receive Timing

The incoming waveform is formed from the transmitted

pulse. The receiver detects the point at which the enve-

lope of the received waveform crosses a set threshold.

See Figure 41.

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 position bin

( o n e of 25 ) whe r e pu lse po sitio n 0 is at

PULSE_POSITION_0 and each subsequent position is

spaced one TIC from the previous one as defined in the

Data Transmit Timing section. Data symbol intervals

are therefore variable and depend on the encoded

data.

Figure 41.

Receive Symbol Timing

Speed Setting

Nominal Data

Rate

PULSE_POSITION_0

Value

(in TICs)

LOW_SPEED

0.7 Mbps

HIGH_SPEED

1.0 Mbps

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

Position 0

Position 1

Position n2

n=0-24

Transmitter

22206B-44

Symbol 1

Symbol 2

Pulse 0

Detected Envelope

Pulse 1

Pulse 2

Begin of receive

Blanking interval

END_DATA_BLANK

Position 1

Position n1

Position 0

Position 1

Position n2

Position 0

Data slice

threshold

Receiver

22206B-45

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