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GM WORLDWIDE ENGINEERING STANDARDS
GMW3110
© Copyright 2010 General Motors All Rights Reserved
February 2010
Page 30 of 336
Table 18: USDT PCI Parameter Definitions
PCI Parameter
Name
Definition
DL
This 4-bit parameter is used to define the usable Data Length (DL) of a single frame (SF)
message. DL can range from 0 to 7 or 6 since the maximum number of useable data bytes
in a single frame (SF) message is 7 if normal addressing is used and 6 if extended
addressing is used. The DL parameter differs from the data length code provided in each
CAN frame as it only reflects the length of the usable data bytes (i.e., It is possible to pad
messages to a fixed maximum length of 8 bytes even though the number of meaningful
bytes is less than 8. In this case, the CAN frame data length code would reflect 8 bytes and
the DL parameter would reflect the usable bytes).
DL High
This 4-bit parameter is combined with the DL Low parameter to allow 12 total bits to define
the data length of a message. The 12 bits allows for a maximum of 4095 bytes in a single
message. DL High occupies the 4 most significant bits of the 12 total message length bits
DL Low
This 8-bit parameter is combined with the DL high parameter to allow 12 total bits to define
the data length of a message. The 12 bits allows for a maximum of 4095 bytes in a single
message. DL Low occupies the 8 least significant bits of the 12 total message length bits.
SN
The Sequence Number (SN) is a 4-bit parameter which is used as a rolling counter to keep
track of the successive consecutive frames of a multi-frame message. The first consecutive
frame (CF) will have a SN value of 1. The SN value for each subsequent frame shall
increment by 1 up to $F and then the next frame shall have a value of 0. This process shall
continue ($0 thru $F, $0 thru $F) until all frames of the multi-frame message have been
successfully transmitted. Flow control frames transmitted during the transmission of the
multi-frame message do not affect this rolling counter.
FS
The Flow Status (FS) is a 4-bit parameter used to indicate to a sender of a multi-frame
message whether it can proceed with the transmission of consecutive frames (CF). A FS
value of $0 indicates ContinueToSend (ConTS)
Note 1
and means that the sender can
continue to transmit the consecutive frames using the Block Size (BS) and Separation
Time (ST) values from the flow control (FC) frame transmitted by the receiver. A FS value
of $1 indicates a Wait (WT) condition. A wait condition means that the sender has to pause
sending consecutive frames until after the receiver transmits a flow control frame with and
FS value of $0 (ConTS). A FS value of $2 indicates an Oveflow (OVFLW) condition where
the receiver
’s buffer is not large enough to receive the incoming message. This flow status
indicates that reception of the message is being terminated.
BS
The Block Size (BS) is a 1-byte parameter that defines the number of consecutive frames
that the sender can transmit before waiting for a flow control frame from the receiver. There
shall never be a FC frame following the last CF frame of a multi-frame message.
ST
min
The Separation Time minimum (STmin) is a 1-byte parameter that defines the minimum
time lapse between consecutive frames. The units on this parameter are 1 ms per count for
values between $00 and $7F. For ST
min
values between $F1 and $F9, the resolution is
100 µs (microseconds) per count where $F1 = 100 µs and $F9 = 900 µs. All other values
are reserved.
WFT
max
(This parameter
is not allowed in
GMLAN
implementation)
The maximum number of Wait Frame Transmissions (WFT
max
) parameter is used to
determine the maximum number of allowed consecutive flow control frames with an FS
value of WT. This parameter is local to each ECU and is not transmitted as part of a flow
control frame. A WFT
max
value of $0 indicates that the ECU is not allowed to send FC.WT
frames. ISO 15765-2 restricts the maximum value of the parameter WFT
max.
Note 1:
International Organization for Standardization (ISO) uses the acronym CTS for this parameter. In this specification it shall be
referred to as ConTS to avoid confusion with Component Technical Specification which uses the acronym CTS in this specification.
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