ISO Response Timing
An important issue is the amount of time needed to get responses over the ISO-9141-2 bus. There can be a long delay from the time a request message is sent, and when response is received. The long delay occurs due to ISO-9141-2 bus activity. The ISO-9141-2 bus does an initialization at 5 bits per second. It is not necessary in some cars, but the specification requires it for the first message in any string of messages. In order to handle this, there are different types of request message command bytes that can be used. One request message command causes the initialization and then sends the request. The other request message command causes only the request to be sent. For faster response, send the first request with initialization and the rest without initialization. Per the ISO-9141-2 specification, if a delay of five (5) seconds or greater occurs without a request being sent, then the initialization must be repeated.
VPW Variable Response
The variable response feature uses the upper 4 bits of the send “byte count” to control how many responses the interface will receive. After receiving the requested number of responses, the interface will not wait 100 msec before reporting the responses. The valid choices for the number of responses are: 1 through 7. If the upper 4 bits of the send “byte count” are 0000, then normal operation will occur (up to 8 responses).
An example RS232 request message for the variable response feature for receiving 2 responses is:
(destination address byte), 02, 23, 01, 02, 03, 00, 00, 00, 00, 00, 00, 00, 00, 2B.
This causes 3 bytes (01 02 03) to be sent on the J1850 VPW bus, and only two responses will be received. Note the 23 for the byte count. A byte count of 63 would have caused 6 responses to have been received.
T16-002 RS232 Communications
The T16-002 and T16-011 interface provides RS-232 opto-isolation between the PC and the diagnostic interface. For most applications, opto-isolation is not needed. To power the opto-isolation the computer communicating with the Diagnostic Interface must keep the DTR line of the RS-232 interface high, and the RTS line of the RS-232 interface low. For proper operation, these lines must be held in these states constantly.
The T16-002 and T16-011 uses a straight through RS-232 cable to connect to a PC. The DB9 connector on the interface uses female pins. To connect the interface to a PC or notebook computer a straight through RS-232 cable with female pins on one end, and male pins on the other end is needed. Opto-isolation is not recommended for use with small hand-held devices like Palm Pilot PDAs or many newer PCs that do not meet the RS232 current requirements for RTS and DTR. A cable longer then 8 feet is also not recommended
The female connector pins of the DB9 on the T16-002 diagnostic interface are:
Pin 2 RX
Pin 3 TX
Pin 4 DTR
Pin 5 SG
Pin 7 RTS
Pins are in the same configuration as on a PC’s COM port. This requires a straight-through RS-232 cable, with Pin 1 going to Pin 1, Pin 2 going to Pin 2, etc.
The T16-002 interface is not recommended for the VW Intelligent Pass-through Mode.