Burkert Type 8056 - Manual (Page 27)

- 27 - RS232_485_ETP_MODBUS_BU_REV02.doc
interface pin (the transmitter enabling signal). This is the main difference with respect to the RS232 interface normally
present on PCs.
In fact, whilst with the RS232 we do not have to worry about the data traffic because it travels on two separate lines
(RXD and TXD), with the RS485 there is only one line with the traffic suitably managed.
The first operation we must do therefore is to activate the transmitter and then send all the data to the communication
port. Next, we have to wait until the last piece of data has gone from the port to the line and only at this point can we
disables the transmitter.
The management software must also absolutely prevent two different devices on the same line from being activated
for transmission at the same time: this would mean that we would have the outputs of the drivers of the two
transmitters connected in parallel, with a consequent overheating of the components and a loss of information. This
eventuality would not break the devices however as they are thermally protected.
On this matter it is necessary to pay extreme attention to the delay between the transmission of the last data byte (the
checksum) and the effective disabling of the transmitter. This delay must be lower than the time equivalent for
transmitting a 10 bit word, because after this time has elapsed, the device addressed will begin its transmission phase.
For the converter with software version 3.50 or greater, in the menu Communication is present the function A.delay.
This setting value in milliseconds, have the function to insert a delay interval for the response from the converter, so
that the transmitter device have enough time to switch in receiver status.
It is also necessary to note that when the line is not piloted by any transmitter, it is in a condition of relatively high
impedance and this may give rise to the reception of characters without the right meaning and possible disturbance.
On this question, the communication management software must activate the transmitter a few seconds before
transmission starts (typically a time of no less than the length of 3 words at the speed used) so as not to give the line
a defined logical status (MARK) and to permit the receivers connected to eliminate any false characters. In the same
way, during reception, it must take any such characters possibly present during line inactivity and rapidly eliminate
them to prevent saturation of the reception buffers, especially so at high speeds
Furthermore, given that the receiver is anyway connected to the transmitter output, in some types of interface it is
necessary to eliminate any characters possibly present, received during the transmission phase, because they are
copies of what has just been sent. See the next section for some programming examples.