RS-232/485 converter has automatic flow control

John Howard, Kw Aware, Ventura, CA -- EDN, 4/12/2001

RS-485 communications can provide longer range and better noise immunity than RS-232, as well as multidrop capability. Because it does not have separate transmit and receive lines, RS-485 requires flow control. RS-232/485 converters often use one of the RS-232 handshaking lines to control direction, but several communications-software packages do not support flow control. The circuit in Figure 1 is an RS-232/485 converter that uses the transmitted signal itself to control the flow. The circuit uses MAX232 and MAX483 interface circuits, IC₁ and IC₂ from Maxim Integrated Products (http://www.maxim-ic.com) to convert between the ICs' respective signal levels and logic levels. Because both ICs invert the signal, the circuit preserves the original sense of the signal. The MAX483 is normally in the receiving mode. When transmission begins, the signal triggers IC₃, the LM555 timer, which in turn toggles IC₂'s DE and RE lines, putting the chip into the transmitting mode.

Q₁, the 2N3906, fully discharges C₁ each time the trigger line goes low, restarting the timing cycle. The values of R₁ and C₁ determine how long IC₃ maintains the transmitting mode after transmission ends. This interval should be long enough such that the converter doesn't switch directions while sending characters containing long sequences of zeros. On the other hand, it shouldn't be so long that the converter misses received characters. The interval T in seconds is T=1/R₁C₁, where R₁ is in ohms, and C₁ is in farads. The flow control responds within a few microseconds after transmission commences, so the converter does not miss any bits at low and medium data rates. The application for this circuit operates at 14,400 bps. Figure 2 shows the timing of the serial and flow-control lines. The entire circuit can fit into a DB-25 (or even a DB-9) back shell.