Burkert Type 8056 - Manual (Page 25)

- 25 - RS232_485_ETP_MODBUS_BU_REV02.doc
byte 23 = 32 chr(32) = ‘ ’ char 20
byte 24 = 49 chr(49) = ‘1’ char 21
byte 25 = 53 chr(53) = ‘5’ char 22
byte 26 = 32 chr(32) = ‘ ’ char 23
byte 27 = 50 chr(50) = ‘2’ char 24
byte 28 = 48 chr(48) = ‘0’ char 25
byte 29 = 48 chr(48) = ‘0’ char 26
byte 30 = 55 chr(55) = ‘7’ char 27
byte 31 = 13 chr(13) = char 28
byte 32 = 10 chr(10) = char 29
byte 33 = 247 checksu
3.3.11. COMUNICATION
The above mentioned communication protocol is used to exchange data between one unit defined as the MASTER and
another unit defined as the SLAVE.
The term MASTER is used here to indicate a device capable of initiating data transmission on its own initiative.
The term SLAVE however, is used to indicate a device capable of transmitting data only in reply to a command
received.
Neither of the two types of devices can receive and transmit contemporaneously: these operations take place in two
different time phases.
Two MASTER type units cannot be connected together because both of the devices would begin to transmit
autonomously and would never succeed in synchronizing themselves.
Analogously, two SLAVE type devices could never exchange information because neither of the two is capable of
initiating communication on its own initiative.
The flow of information in both directions therefore takes place alternately using blocks containing a maximum of 95
words each. A word is composed of 10 bits in all: 1 start bit + 8 data bits + 1 stop bit.
These data blocks are received contemporaneously by all the devices connected along the communication line, but
only the device with the address equal to the one contained in the data block will be the real destined.
The words comprising the block must be sent on the serial line in a continuous mode, without dead time between one
word and the next. The maximum dead time permitted between two words must not exceed the duration of one word.
SLAVE type devices always generate a reply to every message sent to them and correctly received. In the case of
reception errors, there is no reply. In the case of command interpretation errors (incorrect format, missing parameters,
etc.) either a block without data or no reply, may be the reply depending on the case.
The that elapses between the sending of the last data word by the MASTER and the sending of the first reply word by
the SLAVE can be schematized as follows:
last word transmitted by the MASTER
|
|
reply processing: time variable from 0 to 25 milliseconds at the most
| |
| | time equal to 3 complete words
| | |
| | | first word transmitted by the SLAVE as reply
| | | |
.....xxxxx| t0 | t1 |xxxxxx......