ELM327P datasheet(14/51 Pages) ELM

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ELM327

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AT Commands (continued)

a value to it might result in unexpected behaviour. The

actual time when the value for this parameter becomes

effective is determined by its type. Refer to the

Programmable

Parameters

section

for

information on the types.

Note that ‘PP FF ON’ is a special command that

enables all of the Programmable Parameters at the

same time.

PP xx SV yy [ Prog. Param. xx: Set the Value to yy ]

A value is assigned to a Programmable Parameter

using this command. The system will not be able to

use this new value until the Programmable Parameter

has been enabled, however.

PPS

[ Programmable Parameter Summary ]

The complete range of current Programmable

Parameters are displayed with this command (even

those not yet implemented). Each is shown as a PP

value followed by a colon and the value that is

assigned to it. This is followed by a single digit – either

‘N’ or ‘F’ to show that it is ON (enabled), or OFF

(disabled),

respectively.

See

the

Programmable

Parameters section for a more complete discussion.

R0 and R1

[ Responses off (0) or on(1) ]

These commands control the ELM327’s automatic

display of responses. If responses have been turned

off, the IC will not wait for a reply from the vehicle after

sending a request, and will return immediately to wait

for the next RS232 command. This is useful if sending

commands blindly when using the IC for a non-OBD

network application, or simulating an ECU in a basic

learning environment. It is not recommended that this

option normally be used, however, as the vehicle may

have difficulty if it is expecting an acknowledgement

byte and never receives one. The default is R1, or

responses on.

[ Read the input Voltage ]

This initiaties the reading of the voltage present at

pin 2, and the conversion of it to a decimal voltage. By

default, it is assumed that the input is connected to the

voltage to be measured through a 47K

Ω and 10KΩ

resistor divider (with the 10K

Ω connected from pin 2 to

Vss), and that the ELM327 supply is a nominal 5V.

This will allow for the measurement of input voltages

up to about 28V, with an uncalibrated accuracy of

typically about 2%.

SH xx yy zz

[ Set the Header to xx yy zz ]

This command allows the user to manually control

the values that are sent as the three header bytes in a

message. These bytes are normally assigned values

for you (and are not required to be adjusted), but there

may be occasions when it is desirable to change them

(particularly if experimenting with physical addressing).

The value of hex digits xx will be used for the first or

priority/type byte, yy will be used for the second or

receiver/target byte, and zz will be used for the third or

transmitter/source byte. These remain in effect until

set again, or until restored to their default values with

the D, WS, or Z commands.

This command is used to assign all header bytes,

whether they are for a J1850, ISO 9141, ISO 14230, or

a CAN system. The CAN systems will use these three

bytes to fill bits 0 to 23 of the ID word (for a 29 bit ID),

or will use only the rightmost 11 bits for an 11 bit CAN

ID. The additional 5 bits needed for a 29 bit system are

provided through the AT CP command (since they

rarely change).

If assigning header values for the KWP protocols

(4 and 5), care must be taken when setting the first

header byte (xx) value. The ELM327 will always insert

the number of data bytes for you, but how it is done

depends on the values that you assign to this byte. If

the second digit of this first header byte is anything

other than 0 (zero), the ELM327 assumes that you

wish to have the length value inserted in that first byte

when sending. In other words, providing a length value

in the first header byte tells the ELM327 that you wish

to use a traditional 3 byte header, where the length is

stored in the first byte of the header.

If you provide a value of 0 for that second digit of

the first header byte, the ELM327 will assume that you

wish that value to remain as 0, and that you want to

have a fourth header (length) byte inserted into the

message. This is contrary to the ISO 14230-4 OBD

standard, but is in use by many KWP2000 systems for

(non-OBD) data transfer, so may be useful when

experimenting. Support for 4 byte headers has only

been added with v1.2 of the ELM327 IC, and was not

supported in previous versions.

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