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Detailed description

The ST4E1216 and ST4E1240 are a half-duplex differential RS-485 transceiver, suitable for data transmission up

to 40 Mbps. The devices support 64 bus nodes minimum. The receiver and driver are activated through the

dedicated pins DE and RE. Its functional block diagram is shown below.

Figure 44. Block diagram

Device functional modes

The various functional modes of the ST4E1216 and ST4E1240 are detailed in the following function tables.

When the driver enable pin DE is high, the driver is enabled. The differential signal VOD = VA - VB follows the logic

state of the signal present on input D, that is VOD is positive if D is high, VOD is negative when D is low.

When DE logic is low, A and B are set to high impedance, the driver is disabled, and the D signal does not impact

the outputs. The DE pin has an internal pull-down resistor to the ground, which disables the driver by default if left

open.

The D pin has an internal pull-up resistor to VCC, thus when left open while the driver is enabled, output A is high

and B low by default.

Table 11. Driver truth table

INPUT D

ENABLE DE

OUTPUT A

OUTPUT B

Function

H

H

H

L

Actively drive bus high

L

H

L

H

Actively drive bus low

X

L

Z

Z

Driver disabled

X

Open

Z

Z

Driver disabled by default

Open

H

H

L

Actively drive bus high by default

When the receiver enable pin RE is logic low, the receiver is enabled. When the differential input voltage VID =

VA-VB is higher than the positive input threshold VTH+, the receiver output R turns high. When VID is below the

negative threshold VTH-, the output R turns low. And when VID is in-between both threshold voltages, R is

indeterminate.

If RE is logic high, R is high impedance and the VID polarity and voltage do not impact it. The RE pin has an

internal pull-up resistor, thus when left open, the receiver output is disabled by default.

The ST4E1216 and ST4E1240 architecture turns receiver output in a fail-safe high state when the transceiver is

disconnected from the bus (open), the bus lines A, B are shorted together (short), or the bus is not actively driven

(idle).

ST4E1216, ST4E1240

Detailed description

DS14542 - Rev 2

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