Presently have a read-out on an RS485-bus of 1 (one) RS485 kWh-meter, type DDS238-1ZN by means of a Python-script 'assisted' by module minimalmodbus. Adding more kWh-meters means that (prior or during installation) the Slave-Adress of the added kWh-meter has to be shifted from the initial '1'. Such shift-action starts with scanning the RS485-bus to determine where kWh-meter(s) are actually located.
First step is the following simple Python-script
import serial
import minimalmodbus
# Check address 00 = broadcast
instrument = minimalmodbus.Instrument('/dev/ttyAMA0',0) # port name, slave address
instrument.serial.baudrate = 9600
instrument.serial.timeout = 0.5
instrument.debug = True
print instrument
print instrument.read_register(21,0) # registernumber, number of decimals
# Check address 01 = slave 01
instrument = minimalmodbus.Instrument('/dev/ttyAMA0',1) # port name, slave address
print instrument
print instrument.read_register(21,0) # registernumber, number of decimals
# Check address 02 = slave02
instrument = minimalmodbus.Instrument('/dev/ttyAMA0',2) # port name, slave address
print instrument
print instrument.read_register(21,0) # registernumber, number of decimals
The check on Slave-addresses 00 and 01 produces the result (257) as expected, but (due to absence of a device) obviously response on Slave-adress 02 is failing with a time-out.
For further problem-description see http://www.domoticz.com/forum/viewtopic.php?f=31&t=13592#p102901
From test runs, I can see that a time-out occurs. A time-out-signal could be trigger to step-over to check for next slave-address, if I knew the layout of such time-out-signal for a Python-script with minimalmodbus ....
Searching the internet for an alternative, I see all kind of 'wonderful &elaborate' solutions to trap a time-out, but in the perspective of my simple script I am looking for something very basic (preferably a 'one-liner') to enable stepping out of the time-out to check the next slave-address 3, etc.
Looking at those solutions mentioned above, could perhaps the following setup with semi-code be a simple/basic solution? [for which I have been looking in direction of the C-function fread() ]
start of loop
start time-counter
read register from slave address x
N = number of characters received
if time-counter at value t, then
look if N > 0
if N == 0 then
x = x + 1
jump to start of loop
Any hint for a script using Python or MinimalModbus to perform the semi-code, compatible with the first script?
No working solution found/received in this Forum, but a pragmatic, simple remedy has been developed, tested and put in action, as described at http://www.domoticz.com/forum/viewtopic.php?f=14&t=5808&start=20#p113697
Due to the characteristics of it's data-protocol, the remedy is specific for kWh-meter type DDS238-1ZN, but the idea can possibly be applied for other, comparable kWh-meters with RS485-interface.