G-2 Appendix G Date Code 20010518
SEL-2020 Instruction Manual
If you use the SEL-2020 as a slave in a multidrop Modbus configuration, use the Request-To-
Send (RTS) output as your “push-to-talk” signal to “key on” the slave transmitter. Devices that
typically utilize RTS keying include EIA-232 to 4-wire EIA-485 converters and modems bridged
to a shared audio line. The SEL-2020 asserts the RTS line prior to transmitting, executes the
delay established by the SETTLE1 setting, transmits a message, executes the delay established
by the SETTLE2 setting, and deasserts the RTS line. While the SETTLE2 (posttransmit) delay
is executing, the RTS line remains asserted. So, a transmission that occurs during the SETTLE2
delay will be sent without executing the SETTLE1 (pretransmit) delay.
DATA ACCESS CONSIDERATIONS
When you program the Modbus master device to read data, you may access all data in one read
message, or you may access selected data with separate read messages. If you read all data, the
data you read will all correspond to a single data sample from the attached relay. However, if
you read the data in pieces, subsequent reads will not necessarily be from the same data sample.
You can avoid this by using the 05 function code to freeze a copy of the data for reading. This is
discussed in detail later in this appendix.
Data can be stored in either an archive region or a normal database region. If the data are stored
in a normal database region, the data retrieved through Modbus protocol are the newest collected
data from the relays. If the data are stored in an archive region, the data retrieved through
Modbus protocol are the oldest collected data from the relays.
To clear the oldest collected data in an archive region, send the “clear” message (using function
code 05h). After the oldest record is cleared, the next record can be read. The “clear” only
works for data in an archive region. If the data are not stored in an archive region, the SEL-2020
will respond to the “clear” with an exception message containing error code “Illegal Data
Address (02h).”
TIMING
The SEL-2020 will respond to all Modbus requests, if it is going to respond at all, within 0.5
seconds of receiving the request (1.0 seconds if reading from an archive region). To determine
the minimum sampling interval, you must add the maximum time for request and response
messages and for master processing to this response time. If the SEL-2020 receives a Modbus
request before it has finished processing the previous request, it will ignore the new request and
respond to the original one.
The SEL-2020 monitors the elapsed time between receipt of characters. If 3-½ character times
elapse without a new character, then the SEL-2020 ends the message and starts listening for a
new transmission. All messages received by the SEL-2020 must be separated by at least 3-½
character times plus 2.0 ms to ensure there is no confusion between messages. (3-½ character
times is 4.0 ms at 9600 baud.)
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