BIM-UHF Datasheet, PDF (11/17 Pages) Radiometrix Ltd – Low Power UHF Data Transceiver Module

figure 8: fully buffered CMOS interface - digital drive
"RS232" Serial data
It is possible to transmit "RS232" serial data directly at 4.8 to 38.4kb/s baud between a pair of BiM
transceivers in half duplex . The data must be "packetised" with no gaps between bytes. i.e. : The data
must be preceded by >3ms of preamble (55h or AAh hex) to allow the data slicer in the BiM to settle,
followed by 1 or 2 FFh bytes to allow the receive UART to lock, followed by a unique start of message
byte, (01h), then the data bytes and finally terminated by a CRC or check sum. The receiver data slicer
provides the best bit error rate performance on codes with a 50:50 mark:space average over a 4ms
period, a string of FFh or 00h is a very asymmetric code and will give poor error rates where reception is
marginal. Only 50:50 codes may be used at data rates above 20kbit/s.
We recommend 3 methods of improving mark:space ratio of serial codes, all 3 coding methods are
suitable for transmission at 40kbit/s :-
• Method 1 - Bit coding
Bit rate , Max 40kbit/s , Min 250bit/s
Redundancy (per bit) 100% (Bi-phase), 200% (1/3 : 2/3)
Each bit to be sent is divided in half, the first half is the bit to be sent and the second
half, it's compliment. Thus each bit has a guaranteed transition in the centre and a
mark:space of 50:50 . This is Bi-phase or Manchester coding and gives good results,
however the 100% redundancy will give a true throughput of 20kbit/s.
A less efficient, variation of Bi-phase is 1/3 : 2/3 bit coding. Each bit to be sent is divided
into 3 parts, the first 1/3 is a low, mid 1/3 is the data bit and final 1/3 is high. This code
is easy to decode since each bit always starts with a negative transition. This code
should not be sent faster than 100µs per bit (10kbit/s) since the mark/space can vary for
33 to 67%.
Radiometrix Ltd, BiM Data Sheet
page 11