ProSoft Technology PTQ-PDPMV1 - Manual (Page 289)

PTQ-PDPMV1 ♦ Quantum Platform Reference
PROFIBUS DP Master Network Interface Module for Quantum User Manual
ProSoft Technology, Inc. Page 289 of 306
August 12, 2014
Initiator Idle Time (T
ID1
)
After receiving the last character of a telegram, the initiator must wait this amount
of time before it sends the next telegram. The idle time (T
ID
1 ) is the time between
transmission of the last bit of a frame (no acknowledge) and the transmission of
the first bit of the next frame. It is at least the sync time (T
SYN
), plus some safety
margin (T
SM
), but is also calculated as the maximum of these three values: T
SYN
+
T
SM
, min
TSDR
, or T
SDI
(station delay of telegram initiator). The addition of safety
margin (T
SM
) is very important at high baud rates.
Minimum Slave Interval
The minimum slave interval is the minimum time that must expire between two
slave polling cycles in which a slave can exchange data with the Master. To
permit the slave station to be able to respond during every data cycle, it controls
the bus cycle
with this parameter. It is defined in the slave’s GSD file via the
parameter Min_Slave_Interval, which is specified as a 16-bit factor of 100

s
(Min_Slave_Interval = 1 is 100

s). On some older equipment, the PROFIBUS
link was implemented in software (as opposed to within the slave ASIC) and a
typical value was about 2 ms. On newer equipment with modern ASICs, values
down to

100

s can be achieved.
9.8.2 Calculating System Reaction Time
A simplified calculation of system reaction time for a PROFIBUS DP system is
derived from the following parameters:
 T
SDR
(Station Reaction Time)
 The Transmission (Baud) Rate
 The Net Data Length specified
 Min_Slave_Interval (min time between two slave polling cycles)
Example:
One Master and x slaves are connected via PROFIBUS DP. We will use the
maximum available 1536 bytes of output data and 1536 bytes of input data.
These are to transfer to the maximum number slaves allowed, using 1536 input
and output bytes at 12PDPMV1s. Each slave utilizes an SPC3 ASIC.
To calculate the relative bus cycle time for this network:
Let T
MC
= Time of 1 telegram cycle (request telegram + T
SDR
+ slave response).
Let T
BC
= Time of 1 bus cycle (the sum of all telegram cycles).
Given:
T
SYN
= 33 TBits (Bus idle time or PROFIBUS Sync-Time)
T
ID
1 = 75 TBits (SPC3 bus idle time, at 1.5 MB
TID
1 = 36 TBit).
T
SDR
= 30 TBits typical for baud rates ≥ 1.5 MB (SPC3 ASIC).
Min_Slave_Interval = 1 (100

s, from slave GSD file).
Calculate:
1 Tbit (Bit Time) at 12 MB = 1/12000000 bps = 83 ns/bit