Figure 19. Typical Half-Duplex RS-485 Network
3) Don’t overload the cable with too many receivers.
Even though the MAX3291/MAX3292 receives pre-
sent only 1/4-unit load, placing 128 receivers on the
cable will attenuate the signal if spaced out along
the cable and, in addition, cause reflections if
clumped in one spot. The MAX3291/MAX3292 suc-
cessfully drive the cables to correct RS-485/RS-422
levels with 128 receivers, but the preemphasis
effect is significantly diminished.
The MAX3291/ MAX3292 are centered for a load imped-
ance of 54Ω, which corresponds to the parallel combina-
tion of the cable impedance and termination resistors. If
your cable impedance deviates somewhat from this
value, you still get the preemphasis effect (although the
ideal preemphasis time, t
PRE
, may need adjustment).
However, if your cable impedance is significantly differ-
ent, the preemphasis ratio DPER changes, resulting in
significantly less preemphasis. Determine the preempha-
sis ratio versus load by referring to the Driver Differential
Output Voltage vs. R
DIFF
graph in the
Typical Operating
Characteristics
. Read the strong and normal levels from
the graph (remember that the horizontal units are half
your cable impedance) and divide the two numbers to
get DPER (DPER = V
STRONG
/ V
NORMAL
= V
ODP
/ V
OD
).
Figures 19 and 20 show typical network application cir-
cuits with proper termination.
Preemphasis at Low Data Rates
(MAX3292)
At low data rates (<1Msps), preemphasis operation is
not guaranteed because it is highly dependent on the
system power-supply noise. Minimize this noise by
increasing bypass capacitance and using a power
supply with a fast transient response.
