Dodge 2001 Ram 1500, Руководство пользователя

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In addition to reducing wire harness complexity,
component sensor current loads and controller hard-
ware, multiplexing offers a diagnostic advantage. A
multiplex system allows the information flowing
between controllers to be monitored using a diagnos-
tic scan tool. The Chrysler system allows an elec-
tronic control module to broadcast message data out
onto the bus where all other electronic control mod-
ules can “hear” the messages that are being sent.
When a module hears a message on the data bus
that it requires, it relays that message to its micro-
processor. Each module ignores the messages on the
data bus that are being sent to other electronic con-
trol modules.
With a diagnostic scan tool connected into the CCD
circuit, a technician is able to observe many of the
electronic control module function and message out-
puts while; at the same time, controlling many of the
sensor message inputs. The CCD data bus, along
with the use of a diagnostic scan tool and a logic-
based approach to test procedures, as found in the
Diagnostic Procedures manuals, allows the trained
automotive technician to more easily, accurately and
efficiently diagnose the many complex and integrated
electronic functions and features found on today’s
vehicles.
OPERATION - CCD DATA BUS
The CCD data bus system was designed to run at a
7812.5 baud rate (or 7812.5 bits per second). In order
to successfully transmit and receive binary messages
over the CCD data bus, the system requires the fol-
lowing:
•
Bus (+) and Bus (–) Circuits
•
CCD Chips in Each Electronic Control Module
•
Bus Bias and Termination
•
Bus Messaging
•
Bus Message Coding
Following are additional details of each of the
above system requirements.
BUS (+) AND BUS (–) CIRCUITS
The two wires (sometimes referred to as the “twist-
ed pair”) that comprise the CCD data bus are the D1
circuit [Bus (+)], and the D2 circuit [Bus (–)]. The
9
D
9
in D1 and D2 identify these as diagnostic circuits.
Transmission and receipt of binary messages on the
CCD data bus is accomplished by cycling the voltage
differential between the Bus (+) and Bus (–) circuits.
The two data bus wires are twisted together in
order to shield the wires from the effects of any Elec-
tro-Magnetic Interference (EMI) from switched volt-
age
sources. An induced EMI voltage can be
generated in any wire by a nearby switched voltage
or switched ground circuit. By twisting the data bus
wires together, the induced voltage spike (either up
or down) affects both wires equally. Since both wires
are affected equally, a voltage differential still exists
between the Bus (+) and Bus (–) circuits, and the
data bus messages can still be broadcast or received.
The correct specification for data bus wire twisting is
one turn for every 44.45 millimeters (1
3 ⁄
4
inches) of
wire.
CCD CHIPS
In order for an electronic control module to commu-
nicate on the CCD data bus, it must have a CCD
chip (Fig. 5). The CCD chip contains a differential
transmitter/receiver (or transceiver), which is used to
send and receive messages. Each module is wired in
parallel to the data bus through its CCD chip.
The differential transceiver sends messages by
using two current drivers: one current source driver,
and one current sink driver. The current drivers are
matched and allow 0.006 ampere to flow through the
data bus circuits. When the transceiver drivers are
turned On, the Bus (+) voltage increases slightly, and
the Bus (–) voltage decreases slightly. By cycling the
drivers On and Off, the CCD chip causes the voltage
on the data bus circuit to fluctuate to reflect the mes-
sage.
Fig. 5 CCD Chip
BR/BE
ELECTRONIC CONTROL MODULES 8E - 7
COMMUNICATION (Continued)