300-R010-45G Datasheet, PDF (29/36 Pages) Magnetrol International, Inc. – Echotel® Model 300/301 Ultrasonic Non-Contact Transmitter For Level/Volume/Open Channel Flow

PROGRAMMING cont.
CHARACTER FRAMING
The Model 300/301 is factory configured for 9600 baud,
even parity, and 1 stop bit. In serial communication, the bits
are sent out one at a time, least significant bit first. These are
surrounded by a start bit (always low) and a parity bit or a
stop bit (if a parity bit, it will be followed by a stop bit)
(always high).
Stop
Start 0 1 2 3 4 5 6 7
Stop
Parity
Figure 19
MESSAGE FRAMING
All messages start and end with an idle time of 3.5 character
times. For the 300/301 this value is approximately 4 ms at
9600 baud. This will let all the units know that the next byte
to follow is the address field.
3.5 Character
Times ^{11 Bit Times}
Less than 1.5
Character Times
3.5 Character
Times
Address
Function
Data
CRC Check
Figure 20
Because of this method for determining the address byte,
the entire message must be sent in a continuous stream. If
an idle time of more than 1.5 character times occurs before
the completion of the message, the next byte received will
be assumed an address byte and the previously received
bytes will be ignored.
MEMORY MAP
Modbus sets up the data in a memory map. The memory
map is set up in registers that are 2 bytes in length. The read
only registers are numbered form 30001 to 39999. The
read/write registers are numbered from 40001 to 49999. In
order to access these registers; an offset is sent. For exam-
ple, if the data in register 30001 is to be read, an offset of
0000 is sent.
Individual bits in the bit registers are addressed by a sepa-
rated map. The read only bits are numbered from 10000 to
19999 and the read/write bits are numbered from 0 to 9999.
In the XXXX unit, both addresses will access the same bit.
The offset for these bits is the same as the registers. For
example, if 10003 is to be read, and offset of 0002 is sent.
MODEL 300/301 MODBUS MAPPING
A maximum and minimum value of 0 indicates that the value
is Read Only. Care must be taken when disabling relays. The
relay will hold the last state when it is disabled. RS-485 is
used for external communications using Modbus.
The Model 300/301 Transmitter uses the Modbus RTU pro-
tocol. The user must enter the BAUD rate and the parity that
the master will be using. See page 24 for configuration
instructions. Each of these parameters must be set at the
unit before communication can occur.
1. BAUD rate: 1200, 2400, 4800, 9600.
2. Parity: Odd (8, O, 1), Even (8, E, 1), or None (8, N, 2).
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