Chapter 6
Figure 6-6 shows a chip that contains a single RS-485 transceiver. A driver con-
verts a TTL/CMOS voltage at DI to an RS-485 differential voltage, and a
receiver converts an RS-485 differential voltage to a TTL/CMOS-compatible
voltage at RO. The driver and receiver each have an enable input. The Texas
Instruments SN75176B is an early chip that used this configuration and
pinout. Dozens of other chips use the same pinout, often with different perfor-
mance or added features. For example, the MAX3485 uses a 3.3V supply.
Some chips have maximum bit rates lower than RS-485’s 10 Mbps maximum.
As Chapter 7 explains, these chips can provide better signal quality in lines that
don’t require fast bit rates. The MAX3082 has a maximum of just 115,200 bps.
Other chips and their maximum bit rates include the MAX483 at 250 kbps, the
MAX481 at 2.5 Mbps, and National Semiconductor’s DS16F95 at 5 Mbps.
Linear’s LTC1685 supports speeds of up to 52 Mbps.
The MAX1483 has receivers that are 1/8 unit load each, allowing up to 256
transceivers on a bus. To save power, the MAX481 and other chips enter a
low-power shutdown mode when both the driver and receiver are disabled.
Many chips include features to protect the circuits from damage due to high
voltages or ESD. The MAX13444E has ±80V fault protection and allows hot
swapping. Texas Instruments’ SN65LBC184 has transient protection, a receiver
that is 1/4 unit load, and an extended temperature range. The MAX1480A is a
complete isolated interface on a chip.
The MAX3085 and other chips include fail-safe circuits that guarantee a
logic-high receiver output when the receiver’s inputs are open or shorted. Lin-
ear’s LTC2859 has an internal 120Ω termination controlled by a termina-
tion-enable input.Figure 6-6: Many half-duplex RS-485 interface chips use the same configuration and
pinout as the SN75176B.