Endress+Hauser EngyVolt RV15, Руководство по эксплуатации

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EngyVolt RV15 Appendix
45
1
2
3
A0016446
å 17 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 ( ®
ä 43).
• • 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.
11.4.1 Modbus data format
The Modbus Protocol defines the format for the master’s query and the slave’s response. The query
contains the device (or broadcast) address, a function code defining the requested action, any data