ProSoft Technology PTQ-PDPMV1 - Manual (Page 290)

Reference PTQ-PDPMV1 ♦ Quantum Platform
User Manual PROFIBUS DP Master Network Interface Module for Quantum
Page 290 of 306 ProSoft Technology, Inc.
August 12, 2014
In data exchange mode, a telegram header consists of only 9 character bytes. If
we include the bits of the character frame, there are 11 bits for every character
byte (Start Bit + 8bits/char + Stop Bit + Parity). Because only 1 Master is present,
we can ignore the token hold time of token telegrams. Thus, the basic time
required by one telegram cycle (not including data) is obtained by adding the
relevant bus times and the time to transmit the telegram header as follows:
T
MC
4 (in TBits) = 2 * 9(header byte length) * 11 bits/byte + T
SDR
+ T
SYN
+ Tid 1
T
MC
4
= 198 bits + 30 bits + 33 bits + 75 bits = 336 Tbits
T
MC4
(

s) = 336 Tbits * 83 ns/Tbit = 28

s
Thus, 28

s is the basic time required by the telegram header including the bus
times, without accounting for the data. For our example, we must include the
data (1536 bytes Output + 1536 bytes Input - maximum bytes for a PTQ-
PDPMV1). The time for a single telegram cycle with this data included is:
TMC4 = [336 Tbits] + amount of net data = 336 + [(1536 bytes Output + 1536
bytes Input)*(11 bits/byte)] = 34128 Tbits
TMC4 = 34128 * 83 ns/bit
TMC4 = 2.84 ms
To simplify this calculation, you can assume that a basic transfer time of 28 us
plus approximately 1

s per DU data byte (actually 0.83

s/byte) is required to
complete a telegram cycle. The following illustration gives an overview of the
dominant bus times in a telegram cycle (assuming no interference or repetitions).
Note that the slave has a Min_Slave_Interval of

100

s and this dominates the
bus timing for one telegram cycle. However, the Min_Slave_Interval is 100

s
between two polling cycles at the same station. If you have at least 3 stations
present, then the actual transmission time at 12 MB will become the determining
time factor for the bus cycle rather than the Min_Slave_Interval. Refer to the
EN50170 standard for a more detailed calculation of transmission time.
Reference: INTRODUCTION TO PROFIBUS DP, ACROMAG INCORPORATED.