Chapter 6
RS-232’s TX is ±15V, the RS-485 receiver’s input sees a voltage of about ±5V. If
the RS-232’s TX is just ±5V, the RS-485 receiver’s input sees a differential volt-
age of about ±1.6V, which is well above the minimum requirement of ±0.2V.
This interface requires a full-duplex RS-485 or RS-422 interface. If the differ-
ential receiver can accept input voltages as large as the RS-232 driver’s outputs,
you don’t need the voltage divider and can connect the driver and receiver
directly.
Both interfaces invert the signals, so a TTL logic 1 at one end translates to a
TTL logic 1 at the other end. This circuit is usable over short distances at slow
bit rates. If you don’t need to invert the signals, use RS-485’s line A and connect
line B to ground.## +'#
In an RS-485 network, only one driver can be enabled at a time. After sending
data, a node should disable its driver as quickly as possible so the next node to
transmit can enable its driver and start transmitting.
Figure 6-12 shows a transmitted byte and its driver-enable signal. A node can
control the driver-enable line with software or with a hardware-only method.Figure 6-11: Use this circuit for a short link between a device with an RS-485 interface
and one with an RS-232 interface.