Siemens RS-485 - Manual (Page 3)

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Siemens Building Technologies CC1A7865.1en
HVAC Products 31.10.2002
Communication sequence
Both the start and end of a data block are characterized by transmission pauses. The
maximum period of time that may elapse between 2 successive characters is 3 times
the period of time required for the transmission of one character.
The character transmission time (period of time required for the transmission of 1 char-
acter) is dependent on the Baud rate and the type of data format.
Using a data format of 8 data bits, no parity bit and 1 stop bit, the character transmis-
sion time is calculated as follows:
Character transmission time [ms] = 1000 * 9 bits / Baud rate
Data query by the master
Transmission time = n characters * 1000 * x bits / Baud rate
Identification of end of data query
3 characters * 1000 * x bits / Baud rate
Handling of data query by the slave ( £ 250 ms)
Reply by the slave
Transmission time = n characters * 1000 * x bits / Baud rate
Identification of end of reply
3 characters * 1000 * x bits / Baud rate
Identification of the data query or end of the reply with a data format of 10 / 9 bits.
Waiting time = 3 characters * 1000 * x bits / Baud rate
Baud rate Data format [bits] Waiting time [ms]
9.600 9 2.813
19.200 9 1.406
Process
Example