The RS422 streaming interface is a unique feature of the sat-nms LBRX-1MT beacon receiver. The interface reports the measured value in a compressed binary format at the full measurement rate of the receiver (1000 values per second).
The RS422 streaming interface sends each level value as a 2-byte message at 38400 baud, no parity, one stop bit. In this message, the beacon level is contained as a 14 bit unsigned integer value showing the negative beacon level [dBm] in steps if 0.01 dB:
level [14-bit-integer] = - beacon-level [dBm] * 100
The 14 bit value coded this way is capable to represent levels in the range -163.83 .. 0.00 dBm. At the interface, it gets coded into two bytes, each containing 7 bits of the value and one bit to mark the high / low byte of the value.
1st Message Byte
| bit-no | description |
|---|---|
| 7 (msb) | always 1, marks the 1st message byte |
| 6 | bit 13 (msb) of the level value |
| 5 | bit 12 of the level value |
| 4 | bit 11 of the level value |
| 3 | bit 10 of the level value |
| 2 | bit 9 of the level value |
| 1 | bit 8 of the level value |
| 0 (lsb) | bit 7 of the level value |
2nd Message Byte
| bit-no | description |
|---|---|
| 7 (msb) | always 0, marks the 2nd message byte |
| 6 | bit 6 of the level value |
| 5 | bit 5 of the level value |
| 4 | bit 4 of the level value |
| 3 | bit 3 of the level value |
| 2 | bit 2 of the level value |
| 1 | bit 1 of the level value |
| 0 (lsb) | bit 1 (lsb) of the level value |
The following C-code example explains how to retrieve the level value from the binary data received from the RS422 streaming output port of the sat-nms LBRX-1MT:
double get_lbrx_level() {
unsigned char c;
unsigned val;
double level;
while (1) {
c = get_byte_from_lbrx();
if (c & 0x80) {
val = c & 0x7f;
c = get_byte_from_lbrx();
if ((c & 0x80)==0) {
val = (val << 7) | c;
return - (val / 100.0);
}
}
}
}The diagram shown below illustrates the RS422 signal timing and the polarity of the signals at pins 1 and 9 of the M&C connector. To ensure a reliable communication through the RS422 interface, the receive side should terminate the line with 120 Ohms.