RS232 LOW COST RF MODEM
FM-232C-433
MASTER SLAVE COMMUNCATION
When the modems are configured in a master / slave arrangement only the master can initiate
communications. If a number of slaves are used within a network (all with different addresses) then the
master will need to initiate communications with each slave modem in turn. This can be achieved by changing
the address of the master modem from the host controller (see below).
Note: If two modems are both set to master with the same address then either modem can initiate
communications.
TRANSMIT ADDRESS SELECTION
There are two methods of selecting the transmit address;
1. Jumper Link Header (JL) - Through the address jumper links.
2. Host Selected (HS) - By host controller to which the Modem is connected. (See below)
SENDING A NEW TRANSMIT ADDRESS FROM THE HOST CONTROLLER
The host generated address is communicated to the Modem via the standard RS232 data link. In order for the
RF Modem to determine that the data being received from the host controller is a new transmit address (and
not actual data to be transmitted), a specific command must be sent as below;
ZCZCnnnNNNN
where ZCZC is the transmit new address start identifier
and nnn is the new 3 digit address.
and NNNN is the transmit new address end identifier
The address is stored in volatile memory and will be lost on power down. If the modem is set to either master
or slave then it will read its own address jumper links as a default on power up. This enables point to point
communication between two master modems if they both have the same addresses on power up.
CLEARING THE CD
When using the modems in a master slave situation it might be necessary to clear down the CD between the
two modems. This will enable the master to talk to another slave immediately and not have to wait for the 60
seconds until the CD automatically clears down. This is achieved by sending the following string of characters
from the master modem via the standard RS232 data link;
ZCZCCCCCNNNN
BROADCAST MODE
Broadcast mode can be used to send data from the master modem to all of the slave modems within range.
When the master modem’s address is set to zero it is then in broadcast mode. Broadcast mode is one way
communication only and because of this if any of the data is lost in the transmission the slaves will not be able
to request the data to be sent again by the master.
Note: The CD will only be active for 5 seconds in Broadcast mode
DS232_2
June 00
©2000 Reg. No. 227 4001, ENGLAND
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