Company Confidential
27
Raveon Technologies Corp.
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.
For most applications, where one modem talks to one modem, or where all
modems in the system communicate with all other modems in the system, the
Address Mask should stay set to FFFF.
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
M8
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
