7

on the bus is minimized by the bus interface filtering and

control circuitry of the Bus Transceiver IC.

High Current may exist during noise interference and bus

arbitration conditions on the bus. To limit instantaneous cur-

rent direction change and minimize the level of fluctuating

current caused by these conditions, a series resistance is

used in the bus output of the HIP7020. (See Figure 1). A

connection forces a distribution of bus current between

transmitting modules and provides load stability to the IC.

The HIP7020 maintains a uniform and consistent bus wave-

form having specific transition times and propagation delays

to preserve a J1850 analog data stream. Transmitted bus

data is encoded by a HIP7010 Byte Level IO or HIP7030

J1850 microcontroller, (see Figure 1) where “1s” and “0s”

are defined by the length of time in which the bus voltage is

high or low. Precise waveform control is necessary for a

receiving node to accurately decode the difference between

“1s” and “0s” by the time duration of high levels and low lev-

els on the bus. In order to retain bus data integrity, digital

information to be transmitted on the bus is wave shaped and

amplitude controlled in the Bus Transceiver. The transmitted

signal output to the J1850 Bus is a waveform with uniform

edge control and precisely defined voltage levels.

Bus Current and Voltage Control

The Bus Transceiver has a Wave Shaped Voltage Reference

which controls both the Voltage-to-Current Converter and

the Bus Voltage Driver, Q1. The Voltage-to-Current Con-

verter supplies a limited current feed to the collector of Q1.

Together this provides the function of a Voltage Controlled

Current Driver which controls the bus voltage drive level

while supplying limited current to drive the bus load.

The output of the Wave Shaped Voltage Reference is a uni-

form signal which is a scaled waveform of the desired bus

controls the output current driver and is the input to the Volt-

age-to-Current Converter. The internal reference voltage,

by the signal conditions on the bus. This isolation provides

superior Bus stability in the vehicle environment. The bus

waveform supplied to the bus. This is done without feedback

control which is inherently susceptible to oscillation.

Voltage-to-Current Converter

which is proportional to the Wave Shaped Voltage Reference

magnitude and wave shape. This is the maximum current

that can be supplied to the bus and is limited to a value of

30mA typical.

Voltage Controller Current Driver

The Voltage Controller Current Driver, Q1, is the device

which controls the amount of the available current which will

be sourced out to the bus as determined by the Voltage Ref-

erence and allowed by the Voltage-to-Current Converter.

When the Bus voltage is below the Voltage Reference, Q1

allows more current to be sourced out to the J1850 Bus; until

the Bus voltage and the Voltage Reference match or until the

maximum current limit is reached as set by the Voltage-to-

Current Converter.

When the Bus voltage is above the Voltage Reference, Q1

allows less current sourced out to the J1850 Bus; until the

Bus voltage and the Voltage Reference match or until zero

current is being sourced from Q1.

Bus Output Waveform

The bus output waveform shown in Figure 5 is controlled by

the internal Wave Shaper and has a tightly controlled rise

Constant Propagation Time Delay

There is a constant propagation time delay from TX signal

going high or low to the BUS OUT signal (measured at

3.875V). The propagation time delay signals are shown in

Figure 6. The timing to reach the 50% voltage level of the

The propagation time delay from the start of TX input going

The BUS IN input signals, as shown in Figure 6, are

include hysteresis. There is a constant propagation delay for

the Bus to RX receive channel of the Bus Transceiver. The

as measured in reference to the 50% voltage level on the ris-

ing or falling edge of the BUS IN input signal to the rising or

falling edge of the RX output signal.

HIP7020