User's Manual

Page 61 of 83

A Carrier Corp. Company

PART NUMBER T82002163

REV A

to deal with erroneous communication attempts. Movement from layer 0 to layer 1 is designed to

eliminate these issues.

Communication is initiated by transmitting a startup sequence of bytes to the unit. This sequence

is designed to give the unit time to wake-up, respond, and synchronize to the host computer’s baud

rate. Unless all steps are performed correctly, the unit will assume bad communications and return

to what it was doing. The bytes transmitted are as follows.

The host computer initiates communications by transmitting a 0x00 to the unit. When the unit is

able to respond to the attempt it will reply by transmitting a 0x00 back to the host computer. At

this point the baud rate has not been established and the computer will receive back garbage.

Possibly more than one byte. Even though the computer will probably not receive a 0x00, any

returned value(s) should be accepted as an indication to continue.

The unit having sent the 0x00, then sets up to receive a 0x55 hex value from the host computer.

The host computer having received the acknowledging 0x00 (or the best approximation thereof)

will have ≈500msec to transmit the next byte in the sequence before the unit terminates the

communications attempt. The host then transmits a 0x55. The unit will use this pattern of 0’s and

1’s to lock onto the host computer’s baud rate. If any other value is received the unit will

terminate communications and return to what it was doing previously. Communications will then

be disabled until the next temperature is taken. This prevents the unit from constantly responding

to a bad communications attempt.

Having received the 0x55 correctly the unit will respond by again transmitting a 0x00 back to the

host. This time the baud rate is correct and the host computer should correctly receive the 0x00.

The host uses this received 0x00 as an indication to transmit the final 4-byte sequence. Again the

host must transmit the final 4 bytes in less than ≈1.0 second or the unit will terminate what it

thinks is a bad communications attempt. The final sequence of 4 bytes is 0x7A, 0x5E, 0x8A, and

0xA1. This last action accomplishes two goals. It verifies that the unit has indeed locked onto to

the correct baud rate and it acts as a simple password against a bogus communications attempt. If

the unit does not receive the correct 4-byte sequence either due to bad communications or

improper baud rate, it will again end communications and return to what it was doing previously.

The unit having received the final four-byte sequence correctly responds by transmitting a 0x00

for a third and final time to the host. The unit is now set to the Network/Identification or layer 1

communications.

The table below shows the sequence of events required to initiate communications with the unit.

Computer Action Unit Response Action Description

xmit 0x00 Reply 0x00 Get unit’s attention

Rcv something - signal to xmit next byte within .5 seconds

xmit 0x55 Reply 0x00 Unit establishes baud rate and responds

rcv 0x00 signal to xmit final 4 byte sequence within 1 second

xmit 0x7A - Sanity check on baud rate detection

xmit 0x5E -

xmit 0x8A -

xmit 0xA1 Reply 0x00 Sanity check successful – respond

rcv 0x00 -