Acu-Trac® Smart 485 Messaging - SSI Technologies - #1

bits each, which are strung together to create a complete message as shown in the RS-485 Message Construction drawing below.

SSI's Acu-Trac® Smart 485 ultrasonic level transducers broadcast and receive commands over a standard TIA/EIA RS-485 serial data bus enabling the transducers to communicate and share data with other modules located on the data link. The Smart 485 has two types of RS-485 messaging - programming commands and timed data broadcast message (PID 96).

The RS-485 data link is a serial data link meaning that messages are transmitted one bit at time in sequence until the entire message has been completed. Idle states, I.E. the down time in-between messages is used to signal the 485 bus availability to other transmitting nodes within the network.

The Acu-Trac® Smart 485 ultrasonic level transducer is designed to communicate at a rate of 9600 baud. During normal operation, the Acu-Trac® Smart 485 will broadcast a 19 byte volume measurement message every 0.50 seconds consuming approximately 4% of the available data link bandwidth.

For illustration purposes, all the message drawings depicted herein are referenced from the transmitting nodes perspective with the beginning of the message located on the drawing's right hand side and the message end located on the drawing's left.

Messages are formatted into a series of characters, 10

10 Bit Message Character

8 Bit Message Byte

Character Bits 1 - 10

9

Trailing Bit is Always a Logic 1

Lead Bit is Always a Logic 0

Bit time equal to 104.17 usee +/- 5%

Each character within the message string is comprised of a leading bit, or start bit, an 8 bit message byte followed by the trailing bit or stop bit. The start bit is always at a logic low level (0), while the trailing bit is always at a logic high level (1). The message byte is transmitted with the LSB, least significant bit, first followed in sequence with bits 2, 3, 4 and so on. See the above drawing titled Character Definitions.

The bit time for each character bit is 104.17 usecs +/-0.5% which effectively yields a baud rate of 9600 bits per second.

Character N

Character

1

Character 2

Idle Period 1.0417 msec

1 d7 d6 d5 d4 d3 d2 d1 do 0

d7

d6

d5

d4

d3

d2

do

d1

0

0

d7

d6

d5

d4

d3

d2

d1

do

Byte N

Byte 2

Byte 1

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