Siemens SIMATIC NET PROFIBUS manual Delay Time of the Profibus ILM, Transmission Delay Time TTD

6ZB5530–3AC30–0BA1 Infrared Link Modul (ILM)

Converted to bit times this results in the following:

Transmission Rate	Delay Time
in Kbps	in bit times per km
9.6	0.05
19.2	0.10
45.45	0.23
93.75	0.47
187.5	0.94
500.0	2.50
1500.0	7.50

Table 2: Delay Times of Fiber-Optic and RS 485 LAN Cables

To calculate the cable delay time, the maximum cable length in km is multiplied by the delay time corresponding to the transmission rate from the table.

10.2.2Delay Time of the PROFIBUS ILM

In contrast to the propagation time of electrical cables and fiber-optic cables, the delay time through the logic of the PROFIBUS ILM clocked at the transmission frequency is not a physical time constant but depends on the transmission rate. The number of logic levels in series is relevant with each causing a bit time delay.

The delay time per PROFIBUS ILM is 6 bit times from RS 485 input to infrared output when sending and 3 bit times from infrared input to RS 485 output when receiving. The total delay time of the RS 485 input of the sending PROFIBUS ILM to the RS 485 output of the receiving PROFIBUS ILM is therefore 9 bit times.

If several PROFIBUS ILM links are cascaded, the total delay time is the product of 9 bit times of the individual links and the cascading depth.

10.2.3Delay Time of Further Active PROFIBUS Network Components

Please refer to the delay time listed in the documentation of the specific product.

10.2.4Transmission Delay Time TTD

The total delay time of the PROFIBUS network is the sum of all the values calculated in Sections 10.2.1 to 10.2.3.

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