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1 begins processing the request and preparing the response. This takes place within time t2. Then slave
1 sends the response and switches back to reception again immediately after the last character. Slave
2, which "listens in" on the response in the case of an RS485, must wait for time t3 before it can evaluate
the response that is received. Because the device address again does not match, it ignores the response
and switches back to reception. The master cannot send a new instruction until all these times have
elapsed!
No data requests may be made by the master within t1, t2, and t3. Otherwise the IPC 300 would ignore
the instruction or the data on the bus would become invalid because of data collisions.
Time t3 is required by all other slaves on the bus to switch back to reception. (In the case of the RS485,
every IPC 300 connected to the bus also receives the response, so it is essential that the master waits
for this waiting period to elapse before sending a new instruction! Otherwise the other power controllers
will not be ready yet to receive instructions again.)
Minimum response time
In the IPC 300, it is also possible to set a minimum response time of 0 to 500 ms in the configuration.
The IPC 300 will not send back a response before this time elapses (measured since the last character
was received), meaning the duration of t1 + t2 is extended accordingly.
This setting is required for RS485 interfaces in order to:
a) give the master time to switch from "transmit" to "receive"
b) provide the necessary time to other devices that need much longer before evaluating the device ad-
dress of the protocol that is received.
For the RS485 interface, the minimum response time should be configured to ensure that even the
slowest bus user already evaluates the data request from the master and notices that the instruction is
not for them before the response from the IPC 300 begins.
2.5
Error handling
In the event of the following malfunctions, the IPC 300 will not respond to the instruction that was sent:
•
The baud rate and/or data format of the master and IPC 300 do not match
•
The device address of the IPC 300 does not match the one contained in the protocol and is not 255
(a response is always sent if it is 255)
•
The checksum (CRC) is not correct
•
The instruction is defined incompletely or excessively
•
The number of words to read is 0
In these cases, the instruction from the master should be sent again after the timeout time has elapsed.
If the instruction from the master was received without any transmission errors but cannot be processed
for a certain reason, the IPC 300 will respond as provided for in the Modbus protocol – in the response,
the received function code is OR linked to 0x80 as error identification and the error code is sent back in
the data field.
Error codes
•
01: Invalid Modbus function
•
02: Invalid data address
•
03: Data value outside admissible value range
•
04: Error when processing the instruction (e.g., unable to write to flash)
•
06: Currently unable to execute the instruction, please try again
•
08: Write-protected
If the number of words to be read or written exceeds the maximum admissible number (see table of Mod-
bus functions), the IPC 300 will also return the error code 02. When reading float values, the error num-
ber will be entered in the value itself if the value is invalid, i.e., the error code will be transferred as a float
value, rather than the measured value.
2 Protocol description
11
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