communicationmicrocontrollerrs485serial
I have attached the reference circuit I am using for a simple MODBUS (Read Coil) communication application. I am short of Pins on my controller and I do not want to use a dedicated controller pin for RE and DE.
In the above circuit,
is there any way MAX-485-CTRL always remains LOW and is pulled HIGH automatically when there is a signal on DI by using some capacitor combination?
It can probably be made to work, although I wouldn't call it a "production solution"!
One thing you'll need to do is invert the DEN signal as CAN is low-dominant, not high-dominant.
You'll also need to adjust the termination as although both CAN and RS485 are designed to be terminated by a single 120ohm resistor at each end of the bus, the additional bias resistors that RS485 usually includes will pull the bus lines to voltages slightly "either side of the centre-point", whereas CAN needs to idle recessive at ~2.5V on both lines.
IIRC, there is about 0.5V of differential voltage allowed before a dominant bit is detected.
Depending on the margins you have available, this might work for you. It allows the Rx signal to draw the ATTiny Rx input down to 0.6-0.7V, but does not allow current to flow back into the processor. The node "ATTiny VCC" is switched on and off with the processor. If this is 5V (rather than 3.3V) you need to size the resistor so that it doesn't cause backflow problems in the Rx circuit.
If 0.6V is too high to trigger a good LOW in the ATTiny, you might use a low-drop diode like a Schottky or a germanium.
simulate this circuit – Schematic created using CircuitLab
Best Answer
I've looked into this before and after a quick search through Google this is what I found the last time I looked:
simulate this circuit – Schematic created using CircuitLab
Basically when the TX pin is high (idle), the capacitor will slowly charge up until the Schmitt trigger inverter threshold is reached at which point it will switch the MAX485 into receive mode.
When you want to transmit, the start bit (and any subsequent 0 bits) will rapidly discharge the capacitor through the diode switching over the Schmitt trigger inverter causing the MAX485 to go into transmit mode. The during any 1 bits the capacitor will start to charge again, though slowly because of the resistor until it will eventually switch back into receive mode.
You need to select the R-C values such that the capacitor doesn't reach the positive going threshold of the inverter until sufficient time has passed for one byte of data. Each UART byte will always start with a start bit (0) which will keep the MAX485 in transmit mode while you are sending.
To save space you can get Schmitt trigger inverters in single gate packages which are no bigger than a surface mount transistor - the 74LVC1G14 would suffice for the job.