Jean-François Fourcadier, F4DAY

Jean-François FOURCADIER F4DAY Montpellier (France)	ham projects	page d'accueil en français

The local data memory

With what serves the local data memory ?

Very simply, to transmit on the air at the speed of 2 Mbit/s, a message which was preregistered in a nonvolatile memory of type EPROM.

We use a memory of 1 Mbit, inexpensive, whose access time is of 150 NS. The electronics specialists are accustomed to expressing the capacities of the memories in Mbit, the data processing specialists speak more readily about megabytes. Our memory of 1 Mbit represents 128 KB, which will enable us to place a message of identification easily, one or more images compressed in JPEG, a card QSL, a small software to be downloaded, etc...

It will be observed that the transmission of these 128 KB will be carried out into 0,5 S!

The electronic realization of this device is detailed in the "diagrams" section.

How to arrange the data in the EPROM memory ?

The addresses will be traversed by ascending order, from address 00000 to address 1FFFF. A 8 to 1 multiplexer will select in turn the 8 bits composing each byte. By wiring, the choice is made to extract the lowest significant bits first. The cycle is endless, i.e. that once the contents of the last transmitted address, the process is taken again starting from the first address.

To be likely to be able to use the data at the receiver end, we must install packets delimitors.

The reflexion on the protocols of the project doesn't begin yet. However, rather than to do anything, let us inspire in a very provisional way, of an existing structure simple and largely employed : the structure of theEthernet packet IEEE 802.3:

7 bytes

0 1 0 1 0 1 0 1
0 1 0 1 0 1 0 1
0 1 0 1 0 1 0 1
0 1 0 1 0 1 0 1
0 1 0 1 0 1 0 1
0 1 0 1 0 1 0 1
0 1 0 1 0 1 0 1

preamble

1 byte

1 1 0 1 0 1 0 1

packet delimitor

6 bytes 1 1 1 1 1 1 1 1
1 1 1 1 1 1 1 1
1 1 1 1 1 1 1 1
1 1 1 1 1 1 1 1
1 1 1 1 1 1 1 1
1 1 1 1 1 1 1 1 receiver address

6 bytes
.
transmitter address

2 bytes .
. packet length

N bytes .
.

data

4 bytes
.
CRC - error detection

Let's note that a coding length of the data on 2 bytes, limits us today to a packet size of 65535 bytes.

Our various files are prepared using a small hexadecimal editor, then distributed in the 128 KB of useful space. " The free spaces" of the EPROM are left to zero. The contents of the target file are then transferred in the EPROM, using an adequate programmer.

7 bytes	0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1	preamble
1 byte	1 1 0 1 0 1 0 1	packet delimitor
6 bytes	1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1	receiver address
6 bytes	.	transmitter address
2 bytes	. .	packet length
N bytes	. .	data
4 bytes	.	CRC - error detection