RS232 converter <-> RS485 is used to change the voltage levels, complying with these standards. The characteristics of the voltage converter to RS232 levels in transformed, corresponding standard TTL, and then converted into levels, adopted RS485. Converter further has an automatic switch between sending and receiving.
recommendations: device is useful for sending data on long lines a relatively high speed.
- communicate with the computer in half-duplex mode
- Collaboration with eight modules of a series of "blocks RS485»
- communication between the modules in the system on RS485 common two-wire bus
- the possibility of building a network with a maximum length 1200 m
- power supply: 7 … 15VDC
RS232 converter circuitry <-> RS485 is shown in Figure 1.
Due to the time consistent with RS232 interface transmission method shown converter is a matching system voltage levels, meet the necessary transmission types. Microcircuit US1 was used to change the voltage levels from the RS232 serial port. It contains a voltage converter, which converts the input voltage levels into standard TTL (0 … 5 AT) in voltage, necessary for the serial port: -10 B and +10 AT. Besides, signals, coming from the computer connector (-15 AT, +15 AT, processed to TTL levels (0/5 AT.) After processing chip output voltages US1 receives TTL levels, which must then be converted into standard RS485 levels (in theA + in theB> 0,2 AT, in theA-in theB> 0,2 AT. For this purpose, the use of specialized integrated circuit MAX485. This chip is adapted to operate in half-duplex mode. It consists of a linear transmitter and receiver. transmitter output is connected to an integrated circuit and simultaneously with the linear input of the receiver, which means, the transmission line based on the state DE – for transmitter and RE – receiver. In the illustrated configuration, these inputs are connected with each other, however the transition to the lower state of MAX485 chip needs to receive mode, and the transition to a high state provides transfer.
In order not to "block" line, when it is idle, when all the chips, connected to the line, are in receive mode, there are transients, which receivers may misinterpret. To prevent such a situation, resistors R2 … R4 is initially set transmission line to a logical line. In order not to require software, MAX485 controls the mode of operation of the system, Use automatic mode switch. This switch has been implemented on a chip NE555 (US4). She works in a monostable trigger mode, triggered signal, received from the serial port. The appearance of the low state at the output circuit R1OUT US1 (For example, start bit) causes shutdown monovibratora. The diode D2 causes a faster discharge of the capacitor C5 and NE555 immediate response to an input signal circuit. When the start bit is detected, Output OUT chip US4 appears high state, which switches the MAX485 chip to transmit mode and allows you to send data. After sending the appropriate bit MAX485 chip automatically switches to receive mode. Such a control mode of operation frees the line after about 40 ms after completion of sending data, what is important in the case of reading data from the connected modules. After issuing the read module executive response team sent to them about a 100 ms. The power supply used monolithic stabilizer LM7805. capacitors C1 … C4 filtered supply voltage, a diode D1 prevents damage US3 when connected with reverse polarity voltage.
The printed circuit board sample is shown in Figure 2.