Manual Elm 327 (page 45 of 94) (English)

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ELM327

ELM327DSJ Elm Electronics – Circuits for the Hobbyist

www.elmelectronics.com

Multiple PID Requests

The SAE J1979 (ISO 15031-5) standard allows

requesting multiple PIDs with one message, but only if

you connect to the vehicle with CAN (ISO 15765-4).

Up to six parameters may be requested at once, and

the reply is one message that contains all of the

responses.

For example, let us say that you need to know

engine load (04), engine coolant temperature (05),

manifold pressure (0B), and engine rpm (0C) on a

regular basis. You could send four separate requests

for them (01 04, then 01 05, then 01 0B, etc.) or you

could put them all into one message like this:

>01 04 05 0B 0C

to which, a typical reply might be:

00A

0: 41 04 3F 05 44 0B

1: 21 0C 17 B8 00 00 00

The reply is a multiline one, as was discussed in a

previous section. Looking at the reply in detail, the first

line tells us that it is 00A (decimal 10) bytes long, so

we only pay attention to the first ten bytes of the

following lines (and ignore the final three 00’s on the

last line). The first byte is 41, which tells us that the

message is a response to an 01 request.

Following the 41 is the actual information, with the

PID numbers followed by their data bytes. You will

need to know how many data bytes to expect in order

to make sense of it in most cases.

The order in which you ask for the PIDs should not

matter. For example, the previous request might have

been sent as:

>01 0B 04 0C 05

00A

0: 41 0B 21 04 3F 0C

1: 17 B8 05 44 00 00 00

in which case, the responses might be as shown

above (but the order in which the PIDs appear in the

response does not have to match the order in which

they were requested).

Using this technique, you can make more efficient

use of the data bus. The cost is the extra work that you

must do in creating the requests, and in parsing each

response. If you are writing software to do this, the

time initially taken may well be worth it, but if you are

typing requests at a terminal screen, it is very unlikely

that this will be of benefit to you.

Response Pending Messages

The ELM327 has always been a protocol

converter device, receiving data using one protocol,

and sending it with another. Unless the data was

meant to control the ELM327, it was always passed on

without regard to the content of the data. Beginning

with v2.1, that is changing.

The ELM327 now checks each reply to see if it is

a special ‘Response Pending’ message, which may be

used by the ECU to say “Wait, I’m busy.” According to

the SAE J1979 standard, the scan tool should then

wait up to 5 seconds for the requested information to

arrive (and if another ‘Response Pending’ arrives, the

5 second timer should be reset so that the timing starts

over).

The Response Pending reply will always be of the

form:

7F xx 78

where the xx represents the Mode (or SID) that was

being requested. There is no feedback as to the PID

requested.

If bit 2 of PP 2A is set (it is by default), the

ELM327 will support this part of J1979, changing the

timeout to 5 seconds for you if it sees a Response

Pending message. This will only occur for the CAN

and ISO14230 (KWP) protocols as per the standard.

The CAN protocol is not normally limited to only

ISO15765, but may be if you set PP 2A b0 to ‘1’.

Note that the current implementation of this

feature does not keep track of multiple ECUs, some of

which may reply immediately, and some that may reply

with response pending messages. For this reason,

there may conceivably be circumstances when you

may need to filter for only one ECU address when

receiving a Response Pending reply.

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Elm 327 Manual

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