Instructions

Device configuration
akYtec GmbH · Vahrenwalder Str. 269 A · 30179 Hannover · Germany · Tel.: [phone] · www.akytec.de
26
Note: The devices PR110 and PR114 can operate only in the slave mode and only with
the PR-MI485 interface adapter.
In order to organize data exchange in the network over the RS485 interface, a master de-
vice is required. There can be only one master in the network.
Cycle time
The program execution time (cycle time) is automatically adjusted (auto-tuning) depend-
ing on the program complexity. The auto-tuning affects data exchange over Modbus,
since the program execution has a higher priority than request processing. If the program
is large, it can take up all the CPU time and Modbus data exchange will not be performed
correctly.
To avoid this problem, the lower limit for the volume of the Modbus data exchange is re-
served: 50 requests per second. Thus, at least 50 requests per second can be executed
even if the user program is large, and even more if the program is small and the proces-
sor capacity is sufficient. If there is not enough time to poll all devices, the number of re-
quests should be optimized in the user program.
The Query cycle setting depends on the number of polled variables and the polling fre-
quency in the program. It is recommended to set Query cycle to 1 s. In this case, the de-
vice will be able to request up to 50 variables.
Query time
The query time is the actual time it takes the device to run all requests in a queue. If the
queue is short, the device will perform all the request-response cycles and wait for the
specified Query cycle to expire (Fig. 4.5). If the queue is long and the query takes longer
than the specified Query cycle, the device will poll all slaves in the shortest possible
time.
Fig. 4.5 Query time diagram
To minimize the request time, the following is recommended:
 If one or several slaves are not connected or temporarily unavailable, consider to
block the polling in a program or to minimize the Timeout parameter for these de-
vices.
 Consider the number of slaves and the total number of requests when setting the
Query cycle parameter. If the processing time of all requests (query time) takes
longer than Query cycle, the parameter will be ignored.
Polling of multiple devices in the network
Slaves are polled according to the generated queue from the smallest to the largest ad-
dress. In the following example, the slave with the address 8 is polled first, with the ad-
dress 32 last.
Fig. 4.6