Endress+Hauser EngyVolt RV12, Руководство по эксплуатации

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EngyVolt RV12 Appendix
43
1
2
3
A0016446
 11 RS485 signal
1 Start bit
2 Idle State (A) = no signal level on A
3 Idle State (B) = no signal level on
11.3.4 Troubleshooting
This section contains general information. Specific notes for your individual system can be
obtained from the technical support of the software used or from your system integrator.
• Start with a simple network, one master and one slave. With EngyVolt devices this is
easily achieved as the network can be left intact whilst individual instruments are
disconnected by removing the RS485 connection from the instrument.
• Check that the network is connected together correctly as described in (→  42).
• Confirm that the data “transmitted” onto the RS485 is not echoed back to the PC on the
RS232 lines. (This facility is sometimes a link option within the converter). Many PC
based packages seem to not perform well when they receive an echo of the message they
are transmitting.
• Confirm that the address of the instrument is the same as the “master” is expecting.
• If the “network” operates with one instrument but not more than one check that each
instrument has a unique address.
• Each request for data must be restricted to 40 parameters. Violating this requirement
will impact the performance of the instrument and may result in a response time in
excess of the specification.
• Check that the Modbus Protocol mode (RTU or ASCII) and serial parameters (baud rate,
number of data bits, number of stop bits and parity) are the same for all devices on the
network.
• Check that the “master” is requesting floating-point variables (pairs of registers placed
on floating point boundaries) and is not “splitting” floating point variables.
• Check that the floating-point byte order expected by the “master” is the same as that
used by EngyVolt device. (Various software systems can use a number of formats.)
• If possible obtain a second RS232 to RS485 converter and connect it between the RS485
bus and an additional PC equipped with a software package, which can display the data
on the bus. Check for the existence of valid requests.
• Especially for converters with USB connection, experience has shown that data
transmission may be incorrect. Due to the variety of converters on the market a general
type recommendation cannot be made. Use of a RS485/RS232 converter for connection
to the serial PC interface is recommended.
11.4 Modbus protocol general information
Communication on a Modbus Protocol Network is initiated (started) by a “Master” sending
a query to a “Slave”. The “Slave“, which is constantly monitoring the network for queries
addressed to it, will respond by performing the requested action and sending a response
back to the ”Master”. Only the “Master” can initiate a query.
In the Modbus Protocol the master can address individual slaves, or, using a special
“Broadcast” address, can initiate a broadcast message to all slaves. The EngyVolt devices
do not support the broadcast address.