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Modbus Implementation Tank Gauge Transmitter
56 Installation and Operations Manual
9.9 Data Out of Range
When integer data calculated by the ATT is outside the minimum or maximum scale
points or is otherwise out of range, the value returned is that of the maximum Modbus
i 1. For example, if the maximum Modbus integer is 65,534 (as used in our
previous examples) the "data out of range" value is 65,535. The "data out of range"
convention does not apply to the status bits, packed status bits, character data, and
coded data.
The following error conditions will also cause the ATT to return scaled integer values of
maximum Modbus i 1.
•
Off-line Mode
•
Strap Error
•
No CALC
•
API Correction Error
•
CALC Error
This permits a host to detect a serious failure without monitoring other ATT status bits.
9.10 Loopback Test
In accordance with the Modbus specification, function code 8 initiates a loopback test.
The purpose of this test is to check the communication system. It does not affect the
operation of the ATT. The ATT supports only diagnostic code 00. This is a request to
return query data. Upon receiving a loopback message containing this code, the ATT will
echo the message sent by the host. The entire message returned will be identical to the
message transmitted by the host field-per-field. An example of a loopback message is
as follows:
9.11 Hardware Implementation
The ATT uses a 2-Wire EIA485 hardware interface to communicate with the Modbus
master. EIA485 is a high speed differential communications network which allows up to
32 devices to operate on one network. The ATT and Modbus master share a twisted pair
of wires to communicate.
The communication distance EIA485 can reliably travel is dependent on baud rate
(communication speed), wire quality, environmental noise, wiring configuration, and the
number of multi-dropped ATTs. The recommended wire for EIA485 systems is 18-
gauge or larger, shielded, twisted pairs. The shield should be earth grounded at the
Modbus master (control system or computer end). The shield at the ATT should be open.
The ATT- power line acts as a common reference tie to the Modbus master
Host Request
Address
Function
Code
Code HO Code LO Data Data Error
Check
01 08 00 02 F3 26 XX
ATT Response
Address
Function
code
Code HO Code LO Data Data Error
Check
01 08 00 00 F3 26 XX