AM79C901AVCT datasheet(35/90 Pages) AMD

35 / 90 page

Am79C901A

PR ELI M IN AR 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

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

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

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