~1~7~~~
This invention relates to modems, and p r i ularly to wireless
modems, that is, modems designed to transmit digital data over
radio links.
Typically, wireless modems have been relatively unsophisticated,
essentially comprising a combination of a modem similar to those
used to transmit data over voice telephone lines with a radio
transceiver similar to those used for radiotelephony.
Particularly when such modems are used for unattended
communications, as between telemetry systems and a monitoring
station, it has been difficult to monitor the quality of the
radio link or the proper operation of the modem, both during set-
up or maintenance of the system and during actual operation.
There is therefore an increasing demand for an economical
wireless modem which allows for such monitoring. In order that
wireless modems from different sources be interoperable, it is
also desirable that such modems should utilize standardized
communication protocols, enabling modems of different makes to
intercommunicate.
It is an object of the present invention to provide a wireless
modem which provides the capability of providing diagnostic data
over a wireless link both in a command mode used to set up and
test the modem, and during communications, within the framework
of a defined communication protocol, and to deliver such
diagnostic data independently of data being transported, and
which is capable of setting up and obtaining diagnostic data from
a remote modem of similar design.
According to a first aspect of the invention, a modem comprises
a bidirectional serial digital data port, a transceiver, and a
modem receiving digital data for transmission from said interface
and converting it to analog signals for modulating a transmitter
of said transceiver, and receiving and demodulating analog
signals from a receiver of said transceiver to received digital
data provided to said port, said modem unit further comprising
1
a microcontroller connected to provide to said modem data to be
modulated and transmitted and to receive received or demodulated
data from said modem, and switches controlled by said
microcontroller determining whether said modem receives data to
be transmitted from the microcontroller and/or the port and
whether the modem passes received data to the microcontroller
and/or the port.
According to a second aspect of the invention, in a modem unit,
comprising an external data port, a modem, a transceiver, and a
microcontroller controlling the passage of data sequences to be
transmitted from the port through the modem to the transceiver,
and received data sequences from the transceiver through the
modem to the port includes the wireless modem unit, the
improvement wherein the microcontroller includes means to sense
plural operating characteristics of the modem, wherein the
microcontroller may be set up to delay a data string from the
external port while it delivers to the modem a diagnostic header
containing data as to said operating characteristics, and wherein
the microcontroller if so set up is also set up to delay
admission to the port of a data sequence received from said
transceiver and modem while it diverts to itself a diagnostic
header prepended to said received data sequence.
According to a further aspect of the invention, a modem unit,
comprising an internal data port, a modem, a transceiver, and a
microcontroller having an online mode controlling the passage of
data sequences to be transmitted from the port through the modem
to the transceiver, and received data sequences from the
transceiver through the modem to the port, the microcontroller
also having a command mode in which it interprets data sequences
received through the port and executes commands comprised by
those sequences, includes the wireless modem unit the improvement
wherein the microprocessor is configured in command mode to
recognize data sequences from the data port having a particular
first prefix as a command to construct a data sequence
incorporating a command and the address of a remote modem unit
2
?~1~~1~'~
comprised by the original data sequence and itself having a
further prefix identifying it as containing a command, and to
send the constructed sequence through the modem to the
transceiver; and wherein the microcontroller is configured to
recognize data sequences received from the transceiver and the
modem and having such a further prefix as containing either a
command or a response to command.
Other aspects of the invention will become apparent from the
following description of a presently preferred embodiment of the
invention with reference to the accompanying drawing, in which:
Figure 1 is a block schematic diagram of a modem in
accordance with the invention.
A typical modem unit in accordance with the invention comprises
a serial data port 2 through which serial data is passed to and
from the modem unit from an external unit such as a telemetry
device or a supervisory computer, a modem proper comprising a
modulator section 6A and a demodulator section 6B, a transceiver
8, and a microcontroller 10. In the case of a wireless modem, the
transceiver 8 is a radio transceiver as shown.
In accordance with conventional practice, the port 2 has an input
line TXD for data to be transmitted, an output line RXD for
received data, and conventionally designated hand shaking lines,
namely RTS, CTS, DTR and DCD. Data levels on these lines, which
will typically be in accordance with RS232 standards, are
converted to and from TTL levels comparable with the modem logic
by level shifters 4A, 4B and 4C. For convenience, the physical
connection providing this port may also be used to provide lines
CS for transceiver channel selection and received signal strength
indication (RSSI).
The microcontroller 10, which may be of type MC68HC11K4 from
Motorola, comprises a microprocessor, random access memory (RAM),
read only memory (ROM) (in this case electrically erasable ROM
or EEPROM), and various peripheral functions including a number
3
of input and output lines, analog to digital converter channels
and a watchdog function, integrated into a single chip.
Reference to a microcontroller in this specification should not
be considered as excluding usage of a microprocessor and separate
memory and peripheral chips to provide required functions.
A group of output lines from the microcontroller is used to
control tristate buffer gates 12, 14, 16, 18, 20 and 22, which
control respectively a connection between the TTL level TXD
signal and input to an asynchronous to synchronous converter 24A
providing an input to the modulator section 6A (gate 12), a
connection between the input to converter 24A and an input 60 to
the microcontroller programmed to accept serial data (gate 14),
a connection between the input to converter 24A and an output 62
from the microcontroller programmed to output serial data (gate
16), a connection between an output from a synchronous to
asynchronous converter 24B, which receives an output of
demodulator section 6B, to the RXD TTL level signal (gate 18),
a connection from the same output to the input 60 of the
microcontroller programmed to accept serial data (gate 20), and
connection between the serial output line 62 of the
microcontroller and the RXD TTL level signal line (gate 22).
Accordingly, by suitable control of these gates, the TXD line may
be connected to the modem modulator and/or to the microprocessor,
a signal from the microprocessor may be passed to the modem
modulator, and/or to the RXD line, the microprocessor may receive
a signal from the TXD line and/or the modem demodulator, and the
RXD line may receive a signal from the modem demodulator and/or
the microprocessor. These connections permits very versatile
operation of the modem unit under firmware control.
The modem modulator function in the example described is to
receive a synchronous data signal and modulate it to provide an
analog output signal modulated in a selected mode, which in the
embodiment described is MSK (minimum shift keying) when the data
rate is 1200 or 2400 bits/sec and DGMSK (Differential Gaussian
Minimum Shift Keying) at 4800 or 9600 bits/sec. 7-bit scrambling
4
2i 87141
of the signal is applied to provide differential (NRZ1) encoding
of the data when DGMSK is used so as to avoid unfavourable data
patterns introducing undesirable components into the modulated
signal. The modem demodulator provides complementary demodulation
and unscrambling functions. The modulator and demodulator
functions may be implemented by a suitably configured ASIC, or
by utilizing commercially available chips, in conjunction with
resistance ladder 26 constructing an analog output signal from
parallel outputs of the modulator 6A.
The asynchonous to synchronous and synchronous to asynchonous
converters 24A and 24B associated with the modem modulator and
demodulator respectively may conveniently be implemented by the
MC145428 chip from Motorola.
The radio transceiver 8 may be of conventional construction
having regard to the channel frequencies modulation technique and
bandwidth to be utilized. Conveniently, low level modulation and
demodulation in the transceiver are carried out by an RF module,
for example a 3422, 3412 or 3474 module from E. F. Johnson or a
P52 module from Motorola. This module accepts the encoded analog
signal from the ladder 26 after amplification stages 30 and 32
providing filtering and buffering respectively. A gain control
34 controls the amplitude of the signal applied to the
transceiver and thus the deviation of the transmitted signal,
which direction can also be switched by a microcontroller
controlled analog gate 36 to compensate for different bit rates
of the data.
Received signals from the transceiver 8 are applied to an input
filter 38, and an amplifier 40, both having characteristics
switched by analog gates 42, 44 to suit different data rates,
followed by peak detectors 46 and a slicer 48 providing input to
the demodulator 6B.
The microcontroller 10 is also programmed to perform additional
monitoring and control functions through its input and output
5
CA 02187141 2000-03-20
lines. Thus it receives on input lines channel selection signals
which may be applied on lines CS through the same connector that
implements the external port 2 of the modem or by switches (not
shown), and translates these signals into synthesizer control
signals applied through output lines SYN to a synthesizer in the
RF module of transceiver 8. These control signals, including a
synthesizer clock, will be calculated independently according to
whether the transceiver is transmitting or receiving. Typically,
the transceiver will transmit and receive on different
frequencies, and in a network of modems a master unit will
transmit at one frequency and receive at the other, while slave
units will have reversed frequency usage. This means that the
master unit can transmit to any slave unit, and any slave unit
can transmit to the master unit, but the slave units cannot
communicate with one another. A number of input lines to analog
to digital converters incorporated in the microcontroller
interface are connected directly, or through interface circuits
(not shown) so as to monitor transceiver operation, power
supplies 54 to the transceiver required for its proper operation,
and other factors affecting transceiver operation such as a
temperature signal TEMP-M from a temperature sensor 50. Other
parameters sensed include the presence of carrier (CD) from the
transceiver.
The signals whose levels are monitored by the A/D converters in
the microcontroller are power supplies to the transceiver namely
a transmitter power supply (not shown) XMIT_+ and an SW-+ supply
(not shown), as well as a fused input supply F B+ from which all
other supplies to the modem are derived, a 5 volt supply TX-5v
(not shown) to the transmitter logic of the transceiver, a 5 volt
supply to the remaining modem logic, and a supply ANAVCC (not
shown) to the analog circuits of the modem. A received signal
strength index (RSSI) signal from the transceiver is also level
monitored.
Control lines PTT to the power supply 54 determine whether the
transmitter power supply outputs (and thus the transmitter) are
active.
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The functions of the various microcontroller inputs and outputs
are set by a readable configuration register, and an ID number
identifying the particular wireless modem unit is also held in
the EEPROM of the microcontroller for use in identifying
transmissions from the modem.
In operation, the modem provides conventional wireless modem
functions, backwardly compatible with existing wireless modems
using similar bit rates, modulation techniques and radio
frequency parameters, but is characterized by its capability of
both being remotely set up and diagnosed, and by its capability
of providing diagnostic information while on line. For the
purpose of the following description of these capabilities, it
is assumed that a network of at least two wireless modem units
in accordance with the invention and within radio range of one
another has been set up, and that one of these modems, designated
the master, has been selected to source set-up instructions and
receive diagnostic data.
Each modem unit is capable of operating in a normal on-line mode
in which it encodes and transmits data from the port 2 on air or
receives data packets transmitted on air by another modem unit
and decodes and passes data addressed to it to the port 2. It
is also capable of operating in a conventional command mode in
which it receives commands from a computer connected to the port
2 and acknowledges and/or returns data in response to those
commands. Typically, the modem unit is placed in command mode
by raising the DTR handshaking line, and commands are applied to
its port 2 by a computer running radio set-up software (RSS).
The modem of the present invention provides extensions of these
modes which enable remote set-up and diagnostic monitoring. Thus
a modem unit placed in command mode may be sent a command
prefixed by a string which identifies the command as a remote
command, i.e., one to be executed by a remote modem, and
identifies the remote modem unit by its ID number. On
7
217141
recognizing a remote command, the modem unit incorporates it into
a data record or packet identified as a command record and
addressed to the destination modem unit, and transmits it on air.
A listening modem unit which identifies the record as being
addressed to it will recognize the record as a command record,
act on the command, and will transmit an acknowledgement or
response record to the command, addressed to the sending unit and
identified as a response to a command record. A preferred
command protocol is set out in more detail below.
The unit receiving commands from the RSS is considered the master
unit and a remote unit as a slave unit. The master initiates
communication and the slave which is addressed will always
respond in some way if it is functional and within range.
For every command issued to the slave, some response is generated
and sent to the master and hence to the RSS . If no data is being
returned, then a R8 (error report) message is returned with
(assuming no actual error) an OK message (0) within it to
acknowledge the command. The unit may then transmit a test tone
of some sort if that was required by the command.
If data is to be returned to the master, then a sequence of R1
records is generated by the slave, one carrier sequence per
response, then an R9 end of data record is sent . An R8 record
will be issued as for all responses at the very end of the
response. The above R-type (return) records are of the format
already standardized by Motorola, as are the S-type (send)
records referenced below.
The RSS may see echoes of its own commands being sent to the
remote unit and these echoes must be discarded by the RSS. A
timeout of about 3 seconds is used by the RSS software in waiting
for a response from the remote unit.
The variable length on air record consists of the following
elements:
8
....
2187141
72 bits (9 character) packet preamble (DaTaRaDiO in ASCII)
16 bits (2 bytes) length of bytes to follow, high byte first
32 bits (4 bytes) destination ID, binary data, high byte first
32 bits (4 bytes) origin ID, binary data, high byte first
n bytes ASCII text (setup command (no CR), see below)
16 bit CRC for all bytes previous to this word.
The ASCII set-up commands and response set follow below. Those
primarily concerned with diagnostics are marked with an asterisk.
A 16 bit CRC (cyclic redundancy check) is calculated in a
conventional manner using CRC-CCITT implemented in the firmware
of the controller.
Commaad Set
COPTEST Watch dog test
RESET! Forces reset
*RSSI Asks for RSSI level
D & DE Dump whole eeprom
DA Dump whole ram
DC Dump config register
DF Dump factory ID data
*TONExx Ask for test tone xx hex seconds
*BALANCExx Ask for 15 Hz test modulation for balance,
xx secs.
*CARRIER Ask for just a carrier xx hex seconds
*PSUDOxx Ask for xx test data lines in 1 transmission
FACTORYDEFAULTS
Restores EEPROM
user area only
COLDDEFAULTS Restores COMPLETE EEPROM SPACE
ZNWT Zero number of writes for product
*p,R Analog measurement reset
*AS Dump Analog values
*AQ Dump line quality stats
*NR Network online measurements reset
*NS Dump Network online measurement values
SO Motorola SO empty data (checks checksum only)
S1 Load EEPROM with data (checks checksum)
S8 Error/pass code
Sg Calculate new EEPROM checksum now
RO SO format being returned from the radio (dump)
R1 S1 format being returned from the radio (dump)
Rg Error/pass code and done message from radio
R9 Indicates data transfer complete (after some R1'
s)
TO Turn off test mode (temp frequency)
T1 Load synthesizer with test data
? Dump ID string
UNIT Returns only the Unit ID number
WARPEExx Permanently warp all channels by small amount
(optional)
9
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The ANS string is prefixed automatically by the controller
firmware of the remote unit to any response string to be sent to
output port of the master unit before the response is transmitted
by the remote unit.
The following set-up command prefix is used by the RSS to
indicate to the firmware of the master unit that it should pass
the command to a remote unit and will be stripped from the
command before it is sent to the remote unit.
REM00000000 Prefix any command going to the set-up port with
the destination address.
All command strings are generated using only upper case
characters.
The data loaded/returned in S1/R1 records is a function of the
programming of the units and is interpreted accordingly.
Examples are shown below.
Example 1:
Make the local radio transmit one 'fox' test line.
Put the radio into set-up mode.
Send the string "PSUD001<CR>" to the unit.
Unit will respond with R80200FD<CR><LF> and
then transmit one banner and one fox line.
Example 2:
To ask remote unit ID 3 from unit ID 1 to dump analog
values, the command AS with the REM. . .prefix or REM00000003AS<CR>
is presented to the master unit which in turn sends over the air
a command packet with a destination address of 3 and a return
address of 1, as follows:
preamble, length, dest.ID, origin ID, command, crcl6
DaTaRaDiO(Ox000A)Ox00000003(Ox00000001) AS (OxD4C1)
2187141
As an example, the unit 3 could return a byte (in practice more
than 1 byte would probably be returned) and 2 closing packets in
Motorola format:
DaTaRaDiO(0x0019)0x00000001)(0x00000003)ANSR1040DF09965<CR><LF>
Data would normally consist of many R1 transmissions with
incrementing addresses in the R1 record. After all data lines are
sent a R9 record is sent to indicate data transmission is
complete:
DaTaRaDiO(0x0017)(0x00000001)(0x00000003)ANSR9030000FC<CR><LF>
followed by the R8 (done, ready for more commands) record:
DaTaRaDiO(0x0015)(0x00000001)(Ox00000003)ANSR80200FD<CR><LF>
It should be noted that responses have the ANS prefix.
In order to obtain diagnostic data on a continuing basis from
remote modem units without the necessity for a unit to be in set-
up mode, it is a feature of the invention that the modems provide
for each transmit sequence from a modem unit to be preceded by
a fixed length header containing diagnostic data. This can be
achieved by using the microcontroller to control gates 12 to 22
and the handshaking lines of the port 2. When a data sequence
is to be sent through the port 2 to a modem unit for
transmission, as indicated by raising of the RTS line, the gate
16 is activated so that the header containing diagnostic data can
be output from the microcontroller 10 to the converter 24A before
the CTS line is raised and the data to be sent is passed to
converter 24A through gate 12. Likewise, when a data sequence
is received, the gate 20 is activated for the duration of the
header before the DCD line is raised to indicate at the port the
reception of data, and the gate 18 is activated. Data recovered
from the header by the microcontroller firmware may be
transmitted to the port 2 by activating the gate 22. Gate 14 is
used in command mode to allow the microcontroller to receive data
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--. 2187141
from the port 2.
A preferred online protocol for the construction of the
diagnostic header is described further below.
Online diagnostics provide for the transmission of up to 6
statistical data items to other units while normal user traffic
or command packets are being transmitted. This is done by
inserting this record just before the user equipment attached to
the port 2 of the modem is released to send its data . The 6 items
are normally those indicated below but the protocol is expandable
to many other items or formats.
The six normal items are selected from the data monitored by the
A/D inputs of the microcontroller, or data computed by the
microcontroller, and are:
last RSSI reading
last TEMP M reading
last main B+ reading
last computed quality index for the receiver on the channel
last fwd power (if this signal is available from the
transceiver)
last rev power (if this signal is available from the
transceiver)
The protocol multiplexes 6 bytes per transmission. Normally only
1 transmission is necessary to send the required data, but
extensions to more than 6 items could take more packets.
Each receiving unit stores the 6 items while user traffic is
being transmitted.
Either all units must send the diagnostic header or none may as
there is no way for its presence to be predicted by a receiving
unit.
The preferred protocol defines a 72 bit header multiplexing 6
data items as follows:
12
21871 41
7 repeater flag (keep/replace this data)
6 bit 9
bit 8 of ID address
4 ID address greater than 1023 (truncated) flag
5 3.2 spare bits which are to be set to zero
1.0 2 bit code indicating what the 6 bytes are carrying:
00 rssi temp b+ quality, bytes 5, 6 are empty
O1 rssi temp b+ quality, fwd, rev powers
spare, to b determined at a future time
10 11 use first data element as an extension code
15..8 low address bits of source ID for this data which
follows
23..16 first byte data element or extension codes for
bytes 2..5
31..24 second data or first data when extensions are in
use
39..32 third "
47..40 fourth "
55..48 fifth "
63..56 sixth "
71..64 checksum value so the sum of all bytes including
this one remains at zero mod 256.
When an online data sequence is received, a unit will strip off
the 72 bits, process the data if required and present the
remaining user data to the user port.
The quality index is computed as the ratio of a count of the
number of good records received to the total number of times at
least 9 bytes were received with online diagnostic mode enabled.
This provides an indication of the link quality and is returned
by the firmware.
Example 3:
Preamble transmitted before user data.
Ox01 flag/set indicating power is present
0x01 truncated unit id = 1 (id is actually 1 as the
truncated bit not set in flags)
OxC3 rssi in raw (hardware) units
Ox70 temp in raw units
OxBF b+ voltage in raw units
OxFF quality index (ff indicates no packets received yet)
Ox4D forward power in raw units
0x07 reverse power in raw units
OxE9 checksum
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21$7141
The arithmetic sum is 0x400, and as the mod 0x100 of 0x400 is
zero, this packet is deemed good.
The firmware programmed into the EEPROM memory implements the
functions discussed above using conventional techniques used for
programming embedded microcontrollers, and need not be described
in detail particularly since much of it employs routines similar
to those utilized in the control of known modems. Certain
routines relevant to the novel aspects of the applicants' modem
are however set forth in the following pseudocode.
PSEUDO CODE
The following tasks describe in general terms what are the normal
procedure for the transport of command data using a half duplex
link. The command packet protocol follows a basic master slave
sequence, where one unit initiates a sequence as master unit.
Every command issued across the network requires a R8 response
record to be returned. 32 bit addresses permit selection of a
any unit . If data is being returned in response to a command the
R1/R9 data sequence will be followed by the R8 record.
main task:
main init task
run in parallel
rx task
tx task
setup task (local port)
fill queue (incoming user data)
fill queue (incoming secondary command data)
empty queue (outgoing user data)
empty queue (outgoing secondary command data)
forever
main init task:
setup hardware
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2187141
init operating system
init all variables for other tasks
done
setup task:
if (setup-command-found waiting)
parse and execute command
done
tx task:
if (need to transmit)
prepare hardware
key transmitter
wait until turn on (tl) time has passed
if (ONLINE mode)
build preamble
send preamble
wait until all bytes have gone out
wait until at least one synchronous flag time passed
repeat
if (command to-send_pending)
build command packet from incoming secondary data
send command packet
wait until command packet transmitted
if (incoming user data)
send incoming user data
until not (need to transmit)
wait until turn off (t2) time has passed.
unkey transmitter
done
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rx task:
if (carrier found)
prepare hardware
wait until clamp time has passed
if (ONLINE mode)
receive preamble
demultiplex preamble
repeat
receive user data
if (user data is command)
parse and execute command
else fill queue (outgoing user data)
until not (carrier found)
if (delayed command execution)
execute delayed command
done
build preamble:
Build up a multiplexed data preamble record as follows...
7 repeater flag (keep/replace this record)
6 bit 9
5 bit 8 of ID address
4 ID address greater then 1023 (truncated) flag
3..2 spare bits which are to be set to zero
1..0 2 bit code indicating what the 6 bytes are carrying...
00 rssi temp b+ quality, bytes 5,6 are empty
O1 rssi temp b+ quality, fwd, rev powers
10 spare, to be determined at a future time
11 use first data element as an extension code
15.. 8 low address bits of source ID for this data which follows
23..16 first byte data element or extension codes for bytes 2. .5
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31..24 second data or first data when extensions are in use
39..32 third "
47..40 fourth "
55..48 fifth "
63..56 sixth "
71..64 checksum value so the sum of all bytes including this one
remains at zero mod 256.
...and convert to n bytes.
done
receive preamble:
accept exactly n bytes required for preamble
done
demultiplex preamble:
using multiplex record defined above decode data
store as part of locally maintained statistic record
queue to secondary command port if required
done
encode command packet:
The variable length on air record is defined by the following
elements. Data is converted to async data before actually going
on the air.
72 bits (9 character) packet preamble (DaTaRaDiO in ASCII)
16 bits length of bytes to follow, high byte first
32 bits destination ID, binary data, high byte first
32 bits origin ID, binary data, high byte first
n bytes setup class command
17
2~8.7~4i
16 bit CRC for all bytes previous to this word.
(x16 + x15 + x"2 +1)
done
decode command packet:
check that data follows the encoding command packet protocol
if (checks ok)
execute command
done
parse and execute command:
Check which one of the setup commands listed above needs to
be executed.
GRAMMAR for COMMAND and RESPONSE String Assembly:
online transmission [preamble] [user data ; command packet]
regular transmission [user data i command packet]
preamble [<72 bit record defined above>]
user data [<any user data>]
command packet [DaTaRaDiO]
[<16 bit length>]
[<32 bit destination address>]
[<32 bit origin address>]
[answer string ; command string]
[<crc(x~16 + x15 + x~2 +1)>]
answer string [ANS] [<command string>]
commands string [<setup>;<load data radio>;<load data rss>]
[<R8 record]
setup [<commands from list above>]
load data radio [<load data radio>] i [S1 record>] [<S9 record>]
18
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load data RSS [<load data RSS>] i [R1 record>] [<R9 record>]
S1 record Standard Motorola S1 record with checksum.
R1 record Same as S1 but the S is replaced with a
R
so external software can tell echoed data
from outgoing data.
R9 record Same as Motorola S9, end of data mark from
radio.
R8 record Same as R1 record but the 8 denotes error
codes contained inste ad of data items.
S8/R8 records transport one or two byte from the following
codes...
S8/R8 record Error codes:
SE8 NO ERROR 0 OK, command received and accepted
S8E BADCON1MAND 1 Illegal command for addressed unit
S8E BADDATA 2 Non hex char found in S1 strings
S8E BADCKSUM 3 S1 string checksum failed check
S8E BADADDRESS 4 S1 record address out of range
S8E EMPTY ERROR CODE Empty record, no code present
99
S8E RSSI CODE 128 Next byte is rssi (signal
strength) value
S8/R8 record [<S;R>] [802] [<error code>] [<checksum>]
R8 RSSI record [R80380][<signal strength>][<checksum>]
It will be appreciated that the description provided above is
exemplary only of the presently preferred embodiment of the
modem, and changes both to the modem hardware, the protocols
employed and in the operation of the modem may be made within the
scope of the invention, the use of which is not necessarily
restricted to wireless modems.
19
