Multiprotocol interface ICs can be
used to connect a UART to an RS-485 bus
architecture called point-to-point full-duplex (PTP-FD). The PTP connection usually requires drivers and receivers to be
constantly enabled, and therefore "present" on the line. So when such a circuit
board must fit into a point-to-multipoint,
half-duplex system (PTM-HD), the entire
board (usually) must be redesigned.
A simple trick, though, can adapt an
existing PTP-FD board, which provides a
single link between two terminals, for
use in the more complex PTM-HD architecture. PTM-HD involves one master and
multiple slaves, connected by multiple
links. This adapter makes it possible to
reuse the hardware already designed
and manufactured.
A PTP-FD slave board is always ready to
"hear" interrogation/command signals
from the PTM-HD master unit, but it
answers only when it recognizes its own
address. To avoid any effect on the signal-transmission path of the half-duplex side,
the adapter, which operates only on the
slave board's Tx output, maintains itself in
the quiescent state. Only when a slave output starts to transmit does the adapter
become active and transfer that data to
the central unit.
The adapter circuit consists of two
devices (). The RS-485 receiver, IC1a,
is always enabled. It senses the Tx output
of the PTP slave board, drives the timer
(IC2), and enables the RS-485 transmitter
(IC1b), which is typically in a quiescent
state. The triggering occurs when the slave
board Tx output makes a high-to-low transition (a start bit). The timer's En DRV signal
enables the transmitter, maintaining the
En DRV state for a time interval determined by the delay capacitor.
The timer is retriggered by the next high-to-low transition (data bit) of the input signal (). The value of the delay capacitor depends on the Tx input transitions
coming from the PTP slave board, the time
between data packets, and the switching
time between channels (the slave boards
to be addressed).
