STC485E Datasheet, PDF (8/11 Pages) List of Unclassifed Manufacturers – ±15kV ESD-Protected,Slew-Rate-Limited, Fail-Safe,True RS-485 Transceivers

Applications Information
256 Transceivers on the Bus
The standard RS-485 receiver input impedance is 12kO?(one-unit load), and the standard driver can drive up to 32 unit loads.
The STC family of trans -ceivers have a 1/8-unit-load receiver input impe -dance (96kO), allowing up to 256 transceivers to be
connected in parallel on one communication line. Any combination of these devices and/or other RS-485 transceivers with a
total of 32 unit loads or less can be connected to the line.
Reduced EMI and Reflections
The STC485E is slew-rate limited, minimizing EMI and reducing reflections caused by improperly terminated cables. Figure 12
shows the same signal displayed for a STC485E, transmitting under the same conditions.
In general, a transmitter’s rise time relates directly to the length of an unterminated stub, which can be driven with only minor
waveform reflections. The following equation expresses this relationship conservatively:
Length = tRISE / (10 x 1.5ns/ft)
where tRISE is the transmitter’s rise time.
A system can work well with longer unterminated stubs, even with severe reflections, if the waveform settles out before the UART
samples them.
Low-Power Shutdown Mode
Low-power shutdown mode is initiated by bringing both /RE high and DE low. In shutdown, the devices typically draw only 2 nA
of supply current.
/RE and DE may be driven simultaneously; the parts are guaranteed not to enter shutdown if /RE is high and DE is low for less than
50ns. If the inputs are in this state for at least 600ns, the parts are guaran -teed to enter shutdown.
Enable times tZH and tZL in the Switching Characteristics tables assume the part was not in a low-power shutdown state. Enable
times tZH(SHDN) and tZL(SHDN) assume the parts were shut down. It takes drivers and receivers longer to become enabled from
low-power shutdown mode (tZH(SHDN), tZH(SHDN)) than from driver/receiver -disable mode (tZH, tZL).
Driver Output Protection
Two mechanisms prevent excessive output current and power dissipation caused by faults or by bus contention. The first, a foldback
current limit on the output stage, provides immediate protection against short circuits over the whole common-mode voltage range
(see Typical Operating Characteristics). The second, a thermal shutdown circuit, forces the driver outputs into a high-impedance
state if the die temperature becomes excessive.
Line Length vs. Data Rate
The RS-485/RS-422 standard covers line lengths up to 4000 feet. For line lengths greater than 4000 feet, repeater is
required.
Typical Applications
The STC485E transceivers are designed for bidirectional data communications on multipoint bus transmission lines. Figures 14 show
typical network applications circuits.
To minimize reflections, the line should be terminated at both ends in its characteristic impedance, and stub lengths off the main line
should be kept as short as possible.