rpc2a-000-dil Radiometrix Ltd, rpc2a-000-dil Datasheet - Page 18

• Manufacturer Part Number: rpc2a-000-dil
• Description: Uhf Radio Packet Controller
• Manufacturer: Radiometrix Ltd

A Detailed look at the RPC2A's transceiver interface
The RPC2A interfaces to the transceiver using 4 lines :-
TX
TXD
RX
RXD
Note: 1 All lines are 5V CMOS levels
The enable lines - TX & RX
These normally high, active low lines are used to control the transceiver. The RPC2A is a half-duplex
controller thus in normal operation the transceiver is either transmitting or receiving or off. The RPC2A
only enables the TX and the RX at the same time during self test (local loop back).
Transmit Data - TXD
TXD is the serial data to the transmitter, it is held low when the transmitter is not enabled. When the
TX is enabled a synchronous 64kbit/s (15.6 s/bit) serial code is present to modulate the transmitter.
Receive Data - RXD
RXD is a hi-impedance input which is fed with a 'squared-up' (5V logic level) signal from the receivers'
data slicer. The RPC2A contains a very selective, noise immune signal detector and therefor does not
require that the RXD signal be muted in the absence of signal, i.e.. squared-up channel noise may be fed
to the RPC2A when no signal is present.
The RPC2A's Packet Encoder
The packet is made-up of 4 parts:
Preamble
This is a simple 32kHz square wave, the number of cycles being programmed by address 01h of the
EEPROM. The preamble has two functions, the initial portion it is used to allow the data slicer in a
remote receiver to establish the correct slicing point (for the A-433-64-S this takes a minimum of 3ms),
after the receiver has settled, the remaining portion is used by the receiving RPC2A to positively
identify and phase lock onto the incoming the signal (this requires 15 cycles of preamble). The preamble
may extended to wake-up a remote RPC2A in power saving mode.
Frame sync
A 7 bit Barker sequence is used to identify the start of the data. Alternatively if the transmitter is
sending extended preamble (to wake a power saving remote RPC2A) a complimented 7 bit Barker
sequence is sent every 256 preamble cycles as a 'Please Hold The Line' code. An 8th balancing bit is
added after the Barker sequence.
Data
Each byte in the RPC2A's buffer is expanded into a 12 bit symbol prior to sending. The symbol coding
has the following properties :-
Check Sum
Since the receiver checks each symbol for integrity, a simple 8 bit check sum is used to test for overall
packet integrity. This is also coded into a 12 bit symbol prior to transmission.
Radiometrix Ltd, RPC2A
PPENDIX
Perfect 50:50 balance, i.e.. always 6 one's & 6 zero's
There are never more than 4 consecutive one's or zero's. This minimises the low frequency
components in the code and allows fast settling times to be used for the receivers' data slicer.
Minimum Hamming distance = 2, i.e.. each code is different from any other code by a minimum of 2
bits, thus all odd number of bit errors will always be detected.
In general only 256 of 4096 (6.25%) possible codes are valid, i.e.. a 93.75 % probability of trapping a
byte error.
Preamble and the Frame sync codes are not part of the symbol alphabet, a 'clash' signal will cause
immediate termination of the current decode followed by an attempt to lock to the new signal.
output
output
output
input
2 There is no requirement for a carrier/signal detect signal from the transceiver nor for the RXD
output to be muted when no signal is present.
Active low enable for the transmitter.
Serial data to be sent.
Active low enable for the receiver.
Received serial data.
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