Inside RS-485A single-chip solution is the MAX3162E, which contains two RS-232 drivers,
two RS-232 receivers, and an independent RS-485 transceiver. If you’re design-
ing a circuit that might use either interface, the MAX3160E can interface 5V
logic to either RS-232 or RS-485 as selected by the Mode Functionality input
pin..
Sometimes when you have two devices with different interfaces, all you want is
to link them as cheaply and simply as possible. Over short distances, you can
connect a full-duplex RS-485 or RS-422 interface to RS-232 with a few inex-
pensive components.
Figure 6-11 shows a connection between an RS-232 port and an RS-485 port.
RS-485’s B output connects to RS-232’s RX input. Referenced to signal
ground, the B output is near 0V for a logic 1 and near +5V for a logic 0. As
explained in Chapter 4, these voltages don’t meet RS-232’s minimum specifica-
tion, but RS-232 receivers typically use 5V TTL thresholds. RS-485’s A output
is unused and left open.
In the other direction, a voltage divider ensures that the differential input volt-
age doesn’t exceed RS-485’s maximum of ±6V. The receiver’s A input is tied to
ground. The B input sees about 1/3 of the transmitted RS-232 voltage. If theFigure 6-10: This circuit converts between RS-232 and TTL, and between TTL and
RS-485.