Signal and communication specifications
Note: On those vehicles fitted with a diagnostic connector with ground and Battery Supply pins
(e.g. ISO 15031-3 / SAE J1962 connector), all measurements should be referenced to the pins of
that connector. On other vehicles, without ground and supply pins in the diagnostic connector, the
battery posts should be used as reference.
5.1 Signal
For proper operation of the serial communication, both ECU and diagnostic tester shall
correctly determine each logic state as follows (Values for 24 volt systems appear in
parenthesis):
-a logic "0" is equivalent to a voltage level on the line of less than 20 % of VB for
transmitter, 30 % of VB for receiver;
-a logic "1" is equivalent to a voltage level on the line of greater than 80 % of VB
for transmitter , 70 % of VB for receiver.
In addition, the slope times shall be less than 10 % (15 %) of the bit time. The slope
times are defined as the time taken for the voltage to change from 20 % to 80 % of VB,
and from 80 % to 20 % of VB for transmitters.
Voltage levels between 30 % and 70 % of VB may be detected as either logic "0" or
logic "1".
NRZ coding shall be used. The bit time is defined as half the time between the 50 % VB
levels of successive rising or falling edges of alternating "1" and "0" bits.
Figure 2 illustrates the worst case on signal levels. For electrical specifications of
diagnostic testers see 6.2, and for ECUs see 7.2
Figure 2- Signal voltage levels , worst case values.
Keyword Protocol 2000 - Part 1 - Physical Layer, Swedish Implementation Standard
SSF 14230-1 Issue 3 9 of 12
5.2 Communication specification
5.2.1 The configuration is shown schematically in Figure 3
VB
CECU COBW CTE
“1”
or
“0”
“1”
or
“0”
I
ECU
Reading Line K
Vehicle (see section 7) Diagnostic Tester (see section 6)
Ground
Figure 3- Communication schematic
5.2.2 The capacitance contribution of the diagnostic tester and associated cables are
termed CTE The capacitance contribution of the on-board wiring is termed COBW. The
sum of the input capacitance of all the ECUs on the bus is defined thus:
n
CECU = S CECU where n is the number of ECUs on the bus.
i = 1 i
Values for CECU and COBW must be selected such that
CECU +COBW £ 7.2 nF and CTE as specified in 6.2.6
(CECU +COBW £ 5.0 nF and CTE as specified in 6.2.6)
(First values are for vehicles with nominal 12 V supply, values in parentheses are for
24 V vehicle battery voltage systems.)
These values are derived from the circuit resistance and baud rate tolerances (sections
6 and 7), allowed slope times and switching thresholds (section 5.1) and a
communication speed of 10,4 kbaud.
Communication question
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