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Modbus Implementation Tank Gauge Transmitter
54 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 integer + 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 integer + 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
Termination resistors of 120 Ohms are shown at each end of the communication bus to minimize reflections on the
line. Termination resistors may not be necessary at baud rates of 9600 bits per second or slower. Pull-up and pull-
down resistors are also shown at the Modbus master end of the cable. These resistors minimize the affects of noise
when the lines are idle. Only one set of pull-up or pull-down resistors are required per twisted pair cable.
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