1)How many samples per/sec can the autotap take?

2)Is the limitation the OBII connection (baud rate) or the data loger/PC?

3)Is there a dependency on the type of data taken (rpm and timing Vs rpm and A/F)?

thanks, slxrti.

 

1. FAST.
2. OBDII.
3. No.

 

let me clarify item 3..you can sample one parameter, a few, or all of them, each can be independantly selected.

 

Experimental maximum messages per second using the AT1/AT123 V2 interface cable is 80 messages per second. (Note: theoretical maximum for the OBDII data bus is 100 per second).

That means, given a fast enough PC, AutoTap will sample 40 PIDs (one request message, one reply message per PID) per second.

If you select 2 PIDs then you may get about 20 samples per second
If you select 4 PIDs then you may get about 10 samples per second
...
If you select 10 PIDs, then you may get about 4 samples per second.

etc.

There are other scan modes that provide data up to 6 times faster (6 PIDs per message) than what I described above. If you have an LS1 you can use EFILive which implements those other scan modes to provide faster data logging.

Regards
Paul

 

thanks for the reply, I was under the impression that the GM transfer rate was 10.4 Kbytes/sec. Are the sample rate base on this baud rate.

There is also a higher baud rate (I don't remember the exact number) , is this available for usage on GM computer.

slxrti

 

The GM OBDII protocol (SAE J1850 VPW) is nominally 10.4kbps. It is "nominal" because it is a variable pulse width (VPW) signal. Some bits are long pulses, some are short pulses, on average there will be 10,400 bits per second - but depending on the pulse widths it could be slightly faster or slightly slower than 10.4kbps.

4x mode (41.6kbps) is the other data transfer rate that is available. It is usually only used for reflashing which usually requires uploading large blocks of data.

The 100 frames per second figure that I quoted above is based on the following assumptions:
1. 10.4kbps
2. 12 byte frames (the largest J1805 frame is 12 bytes - most data logging frames will be shorter than 12 bytes - usually 8 or 9 bytes)

A frame is made up of:
- a SOF (Start Of Frame) symbol
- 1 to 12, 8 bit data bytes
- an EOF (End Of Frame) symbol

SOF = 200 microseconds
Data bytes consist of 8 bits in different combinations of short and long pulses
- short pulse is 64 microseconds.
- long pulse is 128 microseconds.
- I used an average pulse of 96 microseconds for calculations.
EOF = 280 microseconds.

So a 12 byte frame will take 200+(12*8*96)+280 microseconds:
or 9,696 microseconds which is 9.696 milliseconds - or close enough to 10 milliseconds.

If each message takes 10 milliseconds then you can transmit about 100 messages per second.

Obviously if the messages are only 8 or 9 bytes long then the maximum messages per second will be a little faster.
These figures are just "theoretical" - in practise there are other nodes transmitting data on the same bus which uses up bandwidth, leaving less available for data logging.

Regards
Paul