Burkert Type 8056 - Manual (Page 26)

- 26 - RS232_485_ETP_MODBUS_BU_REV02.doc
Time t0 depends on the type of command sent and the type of converter it is sent to. Time t1 depends on the
communication speed used. A further time t2 - equal to the word length - must be added to these two times. The
integrated circuit receiver needs this time to acquire and extract the data bytes of the first word received.
For the system to work correctly, a MASTER type device must respect the following protocol:
1. block transmission with the code-command;
2. wait for the first reply word for a period of time slightly longer than t0 max. + t1 + t2;
3. reception of the rest of the reply block if arrived or the re-transmission of the block with the code - command.
After a certain number of re-transmissions without reply, an error message may be generated.
The SLAVE reply must always be waited for within a certain time limit (timeout), once this time has elapsed a new
block can be transmitted. If the wait time is too short, a fresh MASTER transmission may take place during the same
time the SLAVE is replying, with the consequent loss of both data blocks.
If the wait time is excessive, the system could become too slow in signalling faults, especially if there are many
instruments connected in the network.
In any case, a timer must be used to indicate the time limit because reply from the SLAVE cannot be trusted with
certainty: in fact, this latter cannot reply in the case of line errors.
If the communication speed is 9600 bps and the format is 10 bits, the length of a word is equal to 1.042 ms. A
reasonable time limit could be calculated thus:
Tlim = t0 max. + t1 + t2 + 1 ms = 25 + 3 x 1.042 + 1 x 1,042 + 1 = 30.17 ms.
IMPORTANT NOTES: There must be a «silence» interval, equal to at least 3 complete words (30 bits), between each data
block and the successive one in transit on the serial line.Before starting transmission, to the network,
activate the transmitter for a time of no less than 3 words (30 bits) and then begin to send the
output characters. This is to give the devices in the network time to eliminate any false characters
received when the network was not in use status.
3.3.12. SUGGESTIONS FOR RS485 INTERFACE USE
When programs are developed which use this type of interface, it is as well to make a few considerations. Firstly, we
should remember that data transits both in reception and in transmission on the RS485 and that when no transmitter
is enabled, the line is in a condition of relatively high impedance.
Physically the RS485 interface is created by a transmitter with a differential output and by a receiver, also with a
differential input. The output of the first is physically connected to the input of the second, according to the scheme
shown below:
TRANS
INPUT
REC.
OUTPUT
TRANS.
ENABLING
RS485 LINE
+
-
+
-
From this scheme we can see how the transmission and reception of different data simultaneously is not possible. Each
function must be activated separately with a specific software command which changes the logical status of an