B&B Electronics ZXT9-IOA-KIT - Manual - Manual (Page 51)

Configuration & Operation
Manual Documentation Number: ZXTx-IO-x-4113m 45
6.1.6 Exception Reporting
This feature provides the ability of reporting possible problems on input devices (AIs and DIs). It is applicable to Modbus
mode only.
For Analog Inputs exception reports will be periodically sent if an input goes outside the low and high thresholds. Once an
input enters the exception state, it must become greater than the low threshold plus the dead band value or less than the high
threshold minus the dead band value in order to come out of the exception state.
In general, the Modbus protocol does not support exception reporting. In a typical Modbus system the Modbus Master sends
a request to a respective Slave device and the slave device will respond with an ACK. Typical Slave data does not contain I/O
addressing data. Any data sent from the Slave to the Master, without the Master first requesting it, will be ignored by the
Master. Therefore, the end user is required to use a custom driver to capture the exception data.
Analog Exception errors are generated when user-defined High or Low limits are exceeded. If an Analog Input value rises
above the High limit, an exception is generated and immediately sent out. Data is updated and retransmitted based on the
Exception Retransmit timer. Exception is transmitted in the timeframe predefined by the user within the allowable range. If the
Exception reporting timeout is set to zero, the exception is sent only once to the Modbus Master.
 Analog Value > HIGH LIMIT = Exception Error.
The High exception error is cleared when the Analog Input value falls below the high limit – the dead band value.
 Analog Value < (HIGH LIMIT – DEAD BAND) = Exception Error Cleared.
If an Analog Input value falls below the LOW limit, an exception is generated and immediately sent out. Data is updated and
retransmitted based on the Exception Retransmit timer.
 Analog Value < LOW = Exception Error.
The Low exception error is cleared when the Analog Input value rises above the low limit + the dead band value.
 Analog Value > (Low + DEAD BAND) = Exception Error Cleared.
6.1.6.1 Sample Modbus Exception Packet
Exception Modbus packets do not follow the typical Modbus protocol. The Xtreme Module is a Slave device and in a typical
system, slave devices do not generate outgoing requests. When the Xtreme Module generates an exception, the Xtreme
Module
will generate a Modbus packet that emulates a “Master Poll”. The exception packet is sent to the Master and does not
require an ACK.
6.1.6.2 Digital Exception Format
Xtreme Module DI exception
01 02 00 01 0E 98 2C
01
Slave Address
02 Function (Read DI’s)
00 I/O Address High (0-15 = Base)
01 Byte Count
0E Digital Inputs (8- DI’s) 1110
98 Checksum High
2C Checksum Low
6.1.6.3 Analog Exception Format
Xtreme Module AI exception
01 04 00 08 00 00 00 00 00 00 00 00 98 2C
01
Slave Address
04 Function (Read AI’s)
00 I/O Address High (0-15 = Base, 16-31=EXP module 1)
08 Byte Count 08
00 Analog Input-1 High Byte
00 Analog Input-1 Low Byte