
70 / 75
ba_aed104_8_e.doc
Baud rate
Transmission time for one ASCII character
2400
4.4 ms
4800
2.2 ms
9600
1.1 ms
19200
0.6 ms
The transmission time for a command string can be calculated approximately with this information. Determine
the number of characters in the command and multiply it with the transmission time. Moreover, the AED has a
processing time for each command. Refer to the command description for these times (times = transmission
plus processing times).
Determining the bus occupancy (Bus Scan):
Frequently it is expedient to determine the bus configuration each time the bus is switched on or if answers of
the AED are not received. The address occupancy of the bus can thus be determined with the aid of the Bus
Scan. A prerequisite for this is that all modules are set to the same baud rate.
1. Initialization of the master interface with the set baud rate of the AEDs
2. Scanning an address with
the command string:
;S00;
(Select Adress)
X;
(output of an invalid command)
3. The AED addressed with the address answers with a ‘?CRLF’, since it does not know the command. If no
answer comes after a time of approx. 100ms, then no AED is present at this address. If
undefined characters or no ? characters are received by the master, then there can be a bus fault or multiple
occupancy of the address. The bus master must react accordingly.
4. Repetition of item 2 with the following addresses 01...31.
If only a few AED are connected and the addresses are known, then the BusScan can naturally only refer to
these addresses. If all AED are determined successfully as bus subscribers, then the identification string of the
AED can possibly be read (measuring point identification and manufacturing number).
The time-out setting for the interface driver of the master is decisive for the speed of the Bus Scan. The select
command requires max. 20...30ms for the output (for 2400 Bd). The AED does not answer to this select
command.
Measured value query in the bus mode:
All AED have been prepared for bus operation with the aid of the preceding chapter and the BusScan has found
all connected AED.
The output format is set with the command COF for the
single measured value query
with the command
MSV?
. The command string is now:
S00; MSV?;
The AED with the address 00 answers with the measured value
S01; MSV?;
The AED with the address 01 answers with the measured value etc.
The following query times result approximately:
Master transmission
:
S00; MSV?;
(9 charact. + 1 charact. pause)
Response time AED:
approx. 40ms
(at ICR2)
AED transmissions:
xxCRLF
(4 characters at COF2)
(10 characters at COF3)