Vaisala WXT520 Guide de l'utilisateur télécharger pdf (Page 161)

Appendix B __________________________________________________________ SDI-12 Protocol

VAISALA______________________________________________________________________ 159

SDI-12 Timing

Figure 31 on page 159 shows a timing diagram for a SDI-12 command

and its response. The tolerance for all SDI-12 timing is ±0.40

milliseconds. The only exception to this is the time between the stop bit

of one character and the start bit of the next character. The maximum

time for this is 1.66 milliseconds, with no tolerance.

- A data recorder transmits a break by setting the data line to spacing

for at least 12 milliseconds.

- The sensor will not recognize a break condition for a continuous

spacing time of less than 6.5 milliseconds. The sensor will always

recognize a break when the line is continuously spacing for more

than 12 milliseconds.

- When receiving a break, a sensor must detect 8.33 milliseconds of

marking on the data line before it looks for an address.

- A sensor must wake up from a low-power standby mode and be

capable of detecting a start bit from a valid command within 100

milliseconds after detecting a break.

- After a data recorder transmits the last character of a command, it

must relinquish control of the data line within 7.5 milliseconds.

0505-218

Figure 31 Timing Diagram

1 2 ... 156 157 158 159 160 161 162 163 164 165 166 ... 170 171

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Vaisala WXT520 Guide de l'utilisateur Page 161