Date Code 20010518 Appendix G G-9
SEL-2020 Instruction Manual
Error Handling
There are a number of errors that an SEL-2020 Modbus port can detect and handle. Framing
errors (message did not have a correct slave address or length) and CRC mismatches will prevent
an SEL-2020 response to the message. If a legitimate message is received, but cannot be
processed, the SEL-2020 will respond with an error response, as indicated in the Message
Framing subsection above. The following is a list of possible exception codes:
01 - ILLEGAL FUNCTION The received function code is not supported.
02 - ILLEGAL DATA ADDRESS Some portion of requested registers is undefined
or invalid. For data writes, this may mean that
the address is read-only. For force single coil
operations, the address is not the beginning of a
valid region.
03 - ILLEGAL DATA VALUE The referenced data value in a force single coil
operation is not valid for the given coil.
04 - FAILURE IN ASSOCIATED DEVICE The port accessed is not currently collecting the
desired data because of improper settings or
because the port is inactive or read from an
empty region.
06 - BUSY, REJECTED MESSAGE The SEL-2020 is unable to respond in a timely
fashion due to internal data access conflicts.
Also, used to indicate insufficient memory for
requested operation.
Master Device Configuration Considerations
Modbus masters are capable of block requesting registers. Block requests of data can be a
problem, as described in the following example. You want 5 registers starting at address 105,
and another 5 registers starting at address 205, and your Modbus master can request up to 125
registers. It will request 105 registers starting at address 105. The SEL-2020 may not have data
defined for all addresses between 100 and 200, and will declare the request invalid. To get these
10 registers, you must alter the maximum registers that your Modbus master can request, or move
the registers to a contiguous area of a user region (using the SET M procedure) and request them
at this new address.
“JOB DONE” EXAMPLES FOR MODBUS
Example #1: Simple Meter Data Access
This example demonstrates the ability of the SEL-2020 to provide data to a Modbus master
device. Set up the SEL-2020, an SEL-321-1 Relay, and a Modbus Master as follows:
1. Connect the SEL-321-1 Relay to an SEL-2020 port (this example uses Port 2). Use the
SEL-C239 (Y type) cable because it handles both communication and IRIG-B. Connect the
communication terminal at the Y end of the cable to a port on the SEL-321-1 Relay.
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