Notes:
1. TEST has 47K pullup to Vcc. TXD has none. EN has a 47K pulldown
2. Vcc must be a 5v regulated supply (4.75 - 5.25v)
3. Pinout is similar to TX1H, with the addition of pin 8 (located 7.62mm on from pin 7)
4. TXD is inverted RS232 at 5v cmos levels. To connect to a true RS232 device, inverting a level shifter must be used (MAX232 type are ideal, but simple NPN transistor switches with pullups often suffice). With typical microcontrollers and uarts, direct connection is possible.
5. The TXL2 provides transmit side of a simplex link, but provided no two devices attempt to transmit at one time, no further restrictions on data transmission need be made, as all transmit timing, valid data identification and datastream buffering is conducted by the unit. There is no 'transmit enable' pin.
6. This unit is compatible with the RXL2 receiver and the TDL2A transceiver
7. A simple addressing structure is included in the datastream. Units may be programmed onto one of eight addresses (all units are supplied set to default ADDR=0 and default channel=0, 433.925MHz)

Condensed specifications

Operating principle of internal modem

To connect to a true RS232 device, inverting RS232-CMOS level shifters must be used. Maxim MAX232 or equivalent are ideal, but simple NPN transistor switches with pull-ups often suffice. With typical microcontrollers and UARTs, direct connection is possible.

Operation: The radio/data stream interface

A 32 byte software FIFO is implemented in both transmit and receive sub-routines. At the transmitting end this is used to allow for the transmitter start up time (about 3ms), while on receiving it buffers arriving packets to the constant output data rate. All timing and data formatting tasks are handled by the internal firmware. The user need not worry about keying the transmitter before sending data as the link is entirely transparent.

For transmission across the link data is formatted into packets, each comprising 3 bytes of data and a sync code. If less than 3 bytes are in the transmit end FIFO then a packet is still sent, but idle state replaces the unused bytes. When the transmit end FIFO is completely emptied, then the transmitter is keyed off .

Operation: Radio interface

Raw data is not fed to the radios. A coding operation in the transmit software, and decoding in the receiver, isolate the AC coupled, potentially noisy baseband radio environment from the datastream.

The radio link is fed a continuous tone by the modem. As in biphase codes, information is coded by varying the duration consecutive half-cycles of this tone. In our case half cycles of 62.5ms and 31.25ms are used.

In idle (or 'preamble') state, a sequence of the longer cycles is sent (resembling an 8KHz tone).

A packet comprises the Synchronising (or address) part followed by the Data part, made up of twelve Groups (of four half cycles duration). Each Group encodes 2 data bits, so one byte is encoded by 4 Groups.

The oscilloscope screen capture shows a single byte being transmitted by TXL2. A BiM2-433-64 transceiver is used to capture the transmitted data.

The character appears on the serial data output (RXD) pin of a RXL2 after about 12.5ms. Busy (STATUS) pin is momentarily set high to indicate the presence of a valid data in the receive buffer of the RXL2.

It can be clearly seen that unlike raw radio modules, RXL2 does not output any noise when there is not any transmission. Data fed into the TXD input of a TXL2 appears at the RXD output of a RXL2 within radio range in the original form it was fed.

Interfacing a microcontroller to TXL2/RXL2

Figure 8: TXL2 and RXL2 interfaced directly to a microcontroller

Serial data should be in the following format:
1 start bit, 8 data bits, no parity, 1 or 2 stop bits
9600bps
0V=low, 5V=high

BUSY pin (in RXL2) can be connected to one of the port pins which can generate an interrupt on low-to-high transition (e.g. RB0/INT pin in the PIC). This can be used to enter a receive sub-routine to download data received from remote TXL2. Therefore, the host does not need to wait in a loop for a packet.

Interfacing RS232 port to TXL2/RXL2

Figure 9: TXL2/RXL2 interfaced to an RS232 port via an RS232 line driver/receiver

Antenna requirements

Three types of integral antenna are recommended and approved for use with the module:

A) Whip: This is a wire, rod, PCB track or combination connected directly to RF pin of the module. Optimum total length is 16cm (1/4 wave @ 433MHz). Keep the open circuit (hot) end well away from metal components to prevent serious de-tuning. Whips are ground plane sensitive and will benefit from internal 1/4 wave earthed radial(s) if the product is small and plastic cased

B) Helical: Wire coil, connected directly to RF pin, open circuit at other end. This antenna is very efficient given it's small size (20mm x 4mm dia.). The helical is a high Q antenna, trim the wire length or expand the coil for optimum results. The helical de-tunes badly with proximity to other conductive objects.

C) Loop: A loop of PCB track tuned by a fixed or variable capacitor to ground at the 'hot' end and fed from RF pin at a point 20% from the ground end. Loops have high immunity to proximity de-tuning.

Limitation of liability

The information furnished by Radiometrix Ltd is believed to be accurate and reliable. Radiometrix Ltd reserves the right to make changes or improvements in the design, specification or manufacture of its subassembly products without notice. Radiometrix Ltd does not assume any liability arising from the application or use of any product or circuit described herein, nor for any infringements of patents or other rights of third parties which may result from the use of its products. This data sheet neither states nor implies warranty of any kind, including fitness for any particular application. These radio devices may be subject to radio interference and may not function as intended if interference is present. We do NOT recommend their use for life critical applications.
The Intrastat commodity code for all our modules is: 8542 6000.

R&TTE Directive

After 7 April 2001 the manufacturer can only place finished product on the market under the provisions of the R&TTE Directive. Equipment within the scope of the R&TTE Directive may demonstrate compliance to the essential requirements specified in Article 3 of the Directive, as appropriate to the particular equipment.
Further details are available on The Office of Communications (Ofcom) web site:
Licensing policy manual