ELM327P datasheet(19/76 Pages) ELM

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ELM327

ELM327DSH

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AT Command Descriptions (continued)

The default setting for the memory function is

determined by the voltage level at pin 5 during power

up (or system reset). If it is connected to a high level

(VDD), then the memory function will be on by default.

If pin 5 is connected to a low level, the memory saving

will be off by default.

[ Monitor All messages ]

This command places the ELM327 into a bus

monitoring mode, in which it continually monitors for

(and displays) all messages that it sees on the OBD

bus. It is a quiet monitor, not sending In Frame

Responses for J1850 systems, Acknowledges for CAN

systems, or Wakeup (‘keep-alive’) messages for the

ISO 9141 and ISO 14230 protocols. Monitoring will

continue until it is stopped by activity on the RS232

input, or the RTS pin.

To stop the monitoring, simply send any single

character to the ELM327, then wait for it to respond

with a prompt character (‘>’), or a low level output on

the Busy pin. (Setting the RTS input to a low level will

interrupt the device as well.) Waiting for the prompt is

necessary as the response time varies depending on

what the IC was doing when it was interrupted. If for

instance it is in the middle of printing a line, it will first

complete that line then return to the command state,

issuing the prompt character. If it were simply waiting

for input, it would return immediately. Note that the

character which stops the monitoring will always be

discarded, and will not affect subsequent commands.

Beginning with v1.3 of this IC, all messages will be

printed as found, even if the CAN auto formatting is on

(CAF1). The previous version of this IC (v1.2) did not

display some illegal CAN messages if the automatic

formatting was on, but now all messages received are

displayed, and if the data format does not appear to be

correct, then ‘<DATA ERROR’ will be shown beside

the data.

If this command is used with CAN protocols, and if

the CAN filter and/or mask were previously set (with

CF, CM or CRA), then the MA command will be

affected by the settings. For example, if the receive

address had been set previously with CRA 4B0, then

the AT MA command would only be able to ‘see’

messages with an ID of 4B0. This may not be what is

desired - you may want to reset the masks and filters

(with AT AR) first.

All of the monitoring commands (MA, MR and MT)

operate by closing the current protocol (an AT PC is

executed internally), then configuring the IC for silent

monitoring of the data (no wakeup messages, IFRs or

CAN acknowledges are sent by the ELM327). When

the next OBD command is to be transmitted, the

protocol will again be initialized, and you may see

messages stating this. ‘SEARCHING...’ may also be

seen, depending on what changes were made while

monitoring.

MP hhhh

[ Monitor for PGN hhhh ]

The AT MA, MR and MT commands are quite

useful for when you wish to monitor for a specific byte

in the header of a typical OBD message. For the SAE

J1939 Protocol, however, it is often desirable to

monitor for the multi-byte Parameter Group Numbers

(or PGNs), which can appear in either the header, or

the data bytes. The MP command is a special J1939

only command that is used to look for responses to a

particular PGN request.

Note that this MP command provides no means to

set the first two digits of the requested PGN, and they

are always assumed to be 00. For example, the DM2

PGN has an assigned

value of 00FECB (see SAE

J1939-73). To monitor for DM2 messages, you would

issue AT MP FECB, eliminating the 00, since the

MP hhhh command always assumes that the PGN is

preceded by two zeros.

This command is only available when a CAN

Protocol (A, B, or C) has been selected for SAE J1939

formatting. It returns an error if attempted under any

other conditions. Note also that this version of the

ELM327 only displays responses that match the

criteria, not the requests that are asking for the PGN

information.

MP hhhh n

[ Monitor for PGN, get n messages ]

This is very similar to the above command, but

adds the ability to set the number of messages that

should be fetched before the ELM327 automatically

stops monitoring and prints a prompt character. The

value ‘n’ may be any single hex digit.

MP hhhhhh

[ Monitor for PGN hhhhhh ]

This command is very similar to the MP hhhh

command, but it extends the number of bytes provided

by one, so that there is complete control over the PGN

definition (it does not make the assumption that the

Data Page bit is 0, as the MP hhhh command does).

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