Company Confidential
28
Raveon Technologies Corp.
Setting A System-Wide Address
If individual addressing is not needed in your system, there are two ways to
ensure it is not used. One way is to set all modems in the system with the same
Unit Address and Destination Address. From the factory, these are both set to
1234, and thus, all modems can communicate with all other modems, using the
address 1234. The advantage of using this system-wide address, is that if there
are other M8T modems on the channel, but in some other system, they probably
will not have the same Unit Address, and thus will not interfere with your system.
To reduce the possibility of data cross-talk, the system implementer may wish to
use a different system-wide address for the Unit Address instead of 1234. There
are over 65,000 addresses available.
The
ATLA
command can be used to set an additional address that the M8T will
listen for.
An alternate way to disable addressing altogether, is set the Address Mask to
0000 (
ATMK
0000 command). This tells the M8T to ignore the address, and
receive every transmission.
Broadcast Transmissions
The double FF is used to identify a broadcast packet. A transmission with a two
digit FF in the first two positions of the destination ID, or in the last two positions
of the destination ID, will be interpreted as a broadcast, and any modem with an
ID that matches the two non-FF digits will receive the data. For example,
sending data with a destination ID of 12FF will be received by any modem with a
unit ID 1200 through 12FF. Sending data with a destination ID of FF34 will be
received by any modem with a unit ID of 0034 through FF34.
The Address Mask
The reason to use hexadecimal digits to represent the unit address, is that along
with the Unit Address programmed into the
M8
, there is an “Address Mask”
programmed into it. The default mask is FFFF. The address mask is used to
determine if a particular data transmission should be received by the modem.
Only in systems where some modems should only talk to certain other modems,
might you want to change the address mask. Whenever data is received over
the air, the Destination Address of the transmission is logically “ANDed” with the
Address Mask in the receiving modem. This is the
Effective Destination Address
.
The receiving
M8T
also ANDs its own Unit Address with its Address Mask. The
result is the
Effective Unit Address
. The
Effective Unit Address
is compared to
the
Effective Destination Address
, and if the two are identical, the data will be
received.
Note: Logically 1
AND
1 = 1, 0
AND
0 = 0, 1
AND
0 = 0, 0
AND
1 = 0
