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FEATURES
FM-232C-433
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Industry Standard RS-232.
Automatic Data Packetising, Error Checking
(Transparent link).
Available at 433MHz (10mW ERP)
Range upto 120 Metres
Network Configurable, User Selectable as Master or
Slave
56 Byte Data Buffer
Broadcast Facility
User Selectable Baud Rate 9600, 4800, 2400, 1200,
Half Duplex
Optional H/W or S/W Flow Control
LED Indication of Data Tx, Rx, CD.
9-30Vdc Power Supply
Requires no radio Licence To Operate
Miniature enclosure 90 x 55 x 40mm
DESCRIPTION
The R.F. Solutions low cost Wireless Modem provides a reliable serial data communications link that can be
used for many data communications applications. The RF Modem is very simple to use and provides an
RS232 ‘invisible link’ up to 9600 baud.
Each Modem may be configured to be either a ‘Master’ or ‘Slave’ to be used in a wireless network. Up to 127
‘slaves’ may be controlled by a single ‘Master’ Modem.
Packetising and encrypting the data as well as error checking with full acknowledgement for all data
transmitted achieve highly reliable communication.
The RF Modem is supplied with an RS232 cable to connect the host and requires only a 9-12Vdc power
supply.
BLOCK DIAGRAM
V
CC
Volatile Mem
9 Way 'D' Type
RS-232
MCU
FM 433MHz
Transceiver
TX
Links
RX
CD
DS232_2
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©2000 Reg. No. 227 4001, ENGLAND
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FUNCTIONAL DESCRIPTION
FM-232C-433
The RF Modem is designed to connect two DTE devices (e.g. PC’s). The RF Modem physical interface is a 9
pin DCE. The host communications RS232 operates at various baud rates, 8 data bits, 1 stop bit, no parity.
The DTE device must either support hardware CTS flow control, XON/XOFF flow control or if hardware flow
control is disabled then the user must be aware of the maximum buffer size of 56 bytes.
(note:
if you use a null Modem to connect a DTE to a DCE using two RF Modems, then the DCE device
should support RTS flow control.)
LED’s
The RF Modem has three LED’s, Tx, Rx, and CD:
CD (Carrier Detect).
The CD LED will flash on/off for 100 mSecs when a RF connection is active. The flashing status
of CD is on a 60 Sec connection timer. After 60 Secs of inactivity the CD LED will automatically
extinguish.
The CD LED is held on when the RF Modem detects another device transmitting (receiving).
This could be its partner, another pair of RF Modems or another RF transmitting device using
the same frequency.
Tx (Transmit).
The Tx LED is illuminated when the RF Modem is currently transmitting an RF packet.
Rx (Receive).
The Rx LED is illuminated when the RF Modem detects a receive packet which is addressed to
that particular RF Modem.
POWER SUPPLY
The RF Modem requires a 9 - 12V @ 100mA power source. On the power jack the centre is +ve connection.
The power switch is a slide switch located on the bottom of the unit.
STANDBY MODE
When not transmitting, the RF Modem continually searches for a carrier signal and valid I/D address, which
match its own. When detected, the RF Modem receives the data, acknowledges back to the transmitting RF
Modem and decodes the data before transmitting it out onto the host RS232 port.
USER CONFIGURABLE JUMPER LINKS
There are several Jumper Links, which the user may set. Please note that these are only read at power up!
ECHO / REPEATER MODE (J1)
In echo mode each data packet that is received is transmitted out immediately after reception. This may be
used to extend the system range.
note:
In Echo Mode, the RF Modem generates an acknowledge signal to the originating RF Modem. The
originating RF Modem will not receive an acknowledge signal from the destination RF Modem if it has been
echoed by another RF Modem.
J1
Option
Open
-
Connected
Echo Mode Selected
DS232_2
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©2000 Reg. No. 227 4001, ENGLAND
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HOST COMMUNICATION SPEED (J2, J3)
FM-232C-433
The data link between the Modem and the host computer may be set using the appropriate jumper links on the
Modem circuit board. It is set according to the following table.
J3
Open
Open
Connected
Connected
J2
Open
Connected
Open
Connected
Baud Rate
1,200
2,400
4,800
9,600
note:
The host communication link is ONLY the speed of communications between Modem and the host PC,
The radio communications speed is controlled by the two Modems which are communicating which is
dependant on many other factors.
MASTER / SLAVE SELECTION (J13)
J13
Open
Connected
Option
Slave Mode
Master Mode
MASTER / SLAVE ADDRESS SELECTION (J6-12)
Used to set the modem’s address when in master or slave operation
Link no
Address bit
J12
6
(MSB)
J11
5
J10
4
J9
3
J8
2
J7
1
J6
0
(LSB)
Note:
if the address links are set to zero (all address jumper links open) then the modem will be in Broadcast
mode (see below for full description)
DATA FLOW CONTROL (J4, J5)
J4
Connected
Open
Open
J5
Open
Connected
Open
Option
Hardware Flow Control (RTS/CTS)
XON/XOFF Flow Control
No Flow Control
J18 - J21 RS232/5V data Input,
these jumpers are used to set the data into the modem, either as RS232 data
or 5V inverted data. Usually set between pins one and two in all cases to select RS232 data in.
DS232_2
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©2000 Reg. No. 227 4001, ENGLAND
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MASTER SLAVE COMMUNCATION
FM-232C-433
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
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©2000 Reg. No. 227 4001, ENGLAND
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ERROR CHECKING / RELIABILITY
Several methods are employed,
a.
Checksum.
b.
I/D & COMMAND
: bits are sent to on each packet.
c.
FM-232C-433
Data Sequencer
: this ensures that the receiving Modem knows the logical sequence of packets so that if
an ‘acknowledge’ only, was destroyed causing re-transmission of a data packed, the receiver knows this
packet has already been received and discards the data.
ANTENNA & RF MODEM POSITIONING
The helical antenna provided will provide adequate range for most applications. For optimum range, follow
these general rules:
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The space around the antenna is as important as the antenna itself.
The optimum position is to locate the antenna so that is protrudes upwards.
If this is not possible due to other design constraints, try to keep the antenna away from other potential
sources of interference such as transformers, batteries and PCB tracks, especially ground planes.
Note:
Personal computers are one of the one of the greatest generators of RF interference. Ensure the RF
Modem is situated as far away as possible.
CE COMPLIANCE
This product is a sensitive electronic instrument and as such it may
temporarily
be affected by Radio
Transmitting Devices being used in close proximity to it (such as Mobile Phones, Walkie Talkies, CB Radios,
Radio Model Controllers and some Microwave Ovens). If it does exhibit symptoms such as display of erratic or
erroneous data, try moving this product away from the source of interference or switching off that source of
interference whilst you are using this product.
CONNECTION INFORMATION
The RF Modem interfaces to the host computer via RS232 connection. A standard DB9 connector as shown in
the diagram below.
PC Serial Port
DTE
(DB25)
8
3
2
20
7
6
4
5
22
RF Modem
DCE
(DB9F)
1
PC Serial Port
DTE
(DB9)
1
RF Modem
DCE
(DB9F)
1
2
3
4
5
6
7
8
9
RTS
CTS
Tx
Rx
DTR
GND
2
3
4
5
6
Tx
2
Rx
3
DTR
GND
5
6
4
7
8
9
RTS
7
CTS
8
9
DS232_2
June 00
©2000 Reg. No. 227 4001, ENGLAND
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