RV-M7 Technical Manual
Notes: This is the default configuration. All units have address 1234, and all modems will talk to all
other modems with address 1234.
Example 2 (a configuration that won’t work)
Sending to Destination Address = 1236
Receiving Unit Address = 1234
Receiving Unit’s Address Mask = FFFF
Result: No data will be received, because the address do not match, and the address mask of FFFF
requires that all digits in the address match. .
Example 3 (able to receive a data from a group, 1230 – 123F)
Sending to Destination Address = 1236
Receiving M7 Unit Address = 1234
Receiving M7 Address Mask = FFF0
Result: Data will be received. 1236 ANDed with FFF0 is 1230. 1234 ANDed with FFF0 is 1230. The
results of the ANDing match, and thus the data will be received.
Example 4 (able to receive from a group, xx34 where xx is any two digits)
Sending Destination Address = 2234
Receiving M7’s Unit Address = 1234
Receiving M7’s Address Mask = 00FF
Result: Data will be received. 2234 AND 00FF equals 0034. 1234 AND 00FF equals 0034, therefore
they match. The results of the ANDing match, and thus the data will be received.
6.5 Error Correction
The M7 modem has a built-in error correction mode, commonly referred to as Automatic Repeat
request (ARQ). It works by checking each reception for errors, and if the data is OK, it sends a short
“ACK” packet back over the air telling the sending station the data was OK.
The M7 modem uses a traditional Carrier Sense Multiple-Access (CSMA) algorithm with
randomized re-try time slots to determine when to re-transmit packets that must be re-sent.
The duration between re-tries increases as the number of attempts increases.
If received data has an error in it or if the receiving modem does not get the data due to
interference, the receiving modem does not send the ACK back, and the sending station will
automatically re-send the data. There are two aspects to configuring ARQ:
a. Enable the ARQ mode in the modem to allow it to transmit ACKs (ATAK 1
command). This command enables the modem to transmit ACK packets.
b. Configure the number of retries the modem should attempt if an ACK packet is not
received back when it sends data (ATRB xx command). For example, if you set
ATRB to 5 with the ATRB 5 command, the modem will wait for an ACK whenever it
sends data. If it receives and ACK back from the modem that it sent data to, it will
do nothing more. But if it does not receive an ACK, it will resend the same data,
trying up to 5 more times.
(10 pages)
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