Varec 2920, Installation And Operation Manual, Page: 167 / 194

MODBUSAppendix A — MODBUS Implementation
Varec, Inc. 157
In addition, messages that are received with a parity error, checksum error, or message format
error will be ignored.
Data Out of Range
When integer data calculated by the 2920 FTT is outside the minimum or maximum scale
points or is otherwise out of range, the value returned is that of the maximum MODBUS
i 1. For example, if the maximum MODBUS integer is 65,534 (as used in our previous
examples) the “data out of range” value is 65,535. The “data out of range” convention does not
apply to the status bits, packed status bits, character data, and coded data.
The following error conditions will also cause the 2920 FTT to return scaled integer values of
maximum MODBUS i 1.
• Off-line Mode
• No CALC
• CALC Error
This permits a host to detect a serious failure without monitoring other 2920 FTT status bits.
Loopback Test
In accordance with the MODBUS specification, function code 8 initiates a loopback test. The
purpose of this test is to check the communication system. It does not affect the operation of
the 2920 FTT. The 2920 FTT supports only diagnostic code 00. This is a request to return query
data. Upon receiving a loopback message containing this code, the 2920 FTT will echo the
message sent by the host. The entire message returned will be identical to the message
transmitted by the host field-per-field. An example of a loopback message is as follows:
Host Request
Address Function
Code Code HO Code LO Data Data Error
Check
01 08 00 02 F3 26 XX
2920 FTT Response
Address Function
Code Code HO Code LO Data Data Error
Check
01 08 00 00 F3 26 XX