ProSoft Technology PTQ-PDPMV1 - Manual (Page 195)

PTQ-PDPMV1 ♦ Quantum Platform Hot Standby Support
PROFIBUS DP Master Network Interface Module for Quantum User Manual
ProSoft Technology, Inc. Page 195 of 306
August 12, 2014
If the local (active) Master fails when it is in possession of the token, the remote
(passive) Master would sense this and reclaim the token after the timeout time
TTO. In other words, the remote (passive) Master will not wait for the entire TTR
(Target Rotation time) to expire before it can send a ping request and detect the
faulty local (active) Master.
TTO is calculated according to the following formula (according to the FDL-layer
specification):
 TTO = 6*TSL + 2*TS*TSL, where TS is the physical address of the remote
(passive) Master.
The formula implies that the physical address of the remote (passive) Master
should be kept as low as possible in order to achieve an optimal time-out time.
This means that the time it takes for the remote (passive) Master to detect an
local (active) Master failure (TFA) is based on two factors: the time it takes to
reclaim the token + the time it takes to send a ping message with retries:
 TFA = TTO + (max_retry_limit + 1)*TSL
The local (active) Master would poll for incoming ping requests every TA ms, and
if no request has been received since the last poll, the COM-bit is cleared. The
poll sequence is asynchronous to the ping sequence, so in the worst case it
might take 2*TA (that is, the last poll took place just before the remote (passive)
Master failed) before the local (active) Master detects a failure.
TA is calculated according to the following formula:
 TA = Max[30, TTR+SM], where SM is a safety margin (10% of TTR).
The formula implies that the minimum time between polls is 30 ms (highest timer
resolution of the RTOS is 5 ms), while the maximum time is proportional to the
Target Rotation time (TTR).
The time it takes for the local (active) Master to detect a remote (passive) Master
failure is not so crucial since no switchover would take place. The local (active)
Master would just carry on operating as a stand-alone Master. Another argument
to keep TA at a reasonable value is that we would like to keep the CPU load as
low as possible since the most important thing for the local (active) Master is to
keep up the pace with its assigned slaves.
The following table gives an example of the discussed timing values for some
baud rates.
Ping Timing Values
The values are based on a PCB PROFIBUS Master Configuration Software
configuration consisting of 96 slaves, where each slave has 16 bytes input data
and 16 bytes output data (that is, max DPRAM size). The bus profile in the bus
parameters set-up is set to "Single Master". The local (active) Master has a
physical address equal to one, which means that the passive one will use
address zero. TTR in the example is doubled compared to the value that the
Master configuration software calculates.