User's Manual

Company Confidential 60 Raveon Technologies Corp.

Result: Data will be received. 1236 ANDed with FFF0 is 1230. 1234 ANDed with FFF0 is

230. 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.

5.6. 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:

1) 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.

2) 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.

The factory default condition is not to send or require ACK packets, so if you wish

to use this mode, program the M7 to transmit ACKs with the ATAK 1 command.

This will cause the modem to send an ACK anytime it receives data from another

modem. Note: If the destination address was a broadcast (FFFF, FFxx, or

xxFF), it will not wait for an ACK.

The number of times it retries to send data that does not get through (does not

get an ACK) is up to the user, but a number of 5 is usually a good compromise.

If after 5 times, the data does not get through, then there probably is something

seriously wrong with the channel or system.

The retried-transmissions are randomly spaced at intervals between

approximately 200mS and 400mS, increasing by 50mS for each attempted