SN75LBC786

QUADRUPLE RS-423-B DRIVER/RECEIVER WITH LOOPBACK

SLLS184 – NOVEMBER 1994

6

POST OFFICE BOX 655303

PARAMETER MEASUREMENT INFORMATION

Vcm

250 mV

250 mV

500

Ω

500

Ω

Vcm = – 7 to 7 V

50%

50%

50%

50%

tPLH

tPHL

Input A

Output Y

Figure 4. Skew Definition Times

Figure 5. Input Balance Test

PRINCIPLES OF OPERATION

In normal operation, the SN75LBC786 functions as four independent drivers and receivers. The loopback mode is

disabled by maintaining a high logic level on the LB input. The receivers consist of differential comparators with

hysteresis and resistive attenuation on the inputs. The resistive attenuation improves the input common-mode range

and also provides additional protection from ESD and over-voltage stress. The differential and common-mode input

impedance are sufficiently high to meet RS-423-B. The balance of the receiver input voltage current characteristics

and bias voltage is such that the receiver remains in the intended binary state when a differential voltage of 500 mV

is applied to the inputs through 500

Ω across the entire common-mode range (see Figure 5).

The drivers meet all RS-423-B specifications. In normal operation, the drivers have built-in current limits and thermal

overload protection. Slew-rate controlling circuitry is included into the design that is adjusted to suit the application

V externally, the transition time defaults to approximately 1.5

µs. The receiver is compatible to the RS-232 with the

use of external input resistors to meet the RS-232 input-resistance specification of 3 k

Ω to 7 kΩ.

Taking an individual LB input low activates the loopback mode in the corresponding driver/receiver pair. This causes

the output from that driver to be fed back to the input of its receiver through dedicated internal-loopback circuitry. Data

from the receiver output can then be compared, by a communication system, with the data transmitted to the driver

to determine if the functional operation of the driver and receiver together is correct.

In the loopback mode, external data at the input of the receiver is ignored and the driver does not transmit data onto

the line. Extraneous data is prevented internally from being sent by the driver in the loopback mode by clamping its

output to a level below the maximum interface voltage, – 5 V, or the EIA-423-B marking state. Below this marking level,

a reduced 1.5-V output amplitude is used at the driver output. This signal is detected by an on-chip loopback

comparator and fed to the input stage of the receiver to complete the loop.

Line faults external to the SN75LBC786 are detected in addition to device failures. These line faults include short

circuits to ground and to external supply voltages. The loopback mode should be entered only when the driver output

is low, that is, the marking condition. It is recommended that loopback not be entered when the driver output is in a

high state as this may cause a low-level, nondamaging oscillation at the driver output.