Varec 4000 manual Data Out of Range, Loopback Test, Hardware Implementation

Modbus Implementation Tank Gauge Transmitter

9.9Data 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.

9.10Loopback 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:

Host Request

Address	Function	Code HO	Code LO	Data	Data	Error
	Code					Check
01	08	00	02	F3	26	XX
ATT Response
Address	Function	Code HO	Code LO	Data	Data	Error
	code					Check
01	08	00	00	F3	26	XX

9.11Hardware 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.

54 Installation and Operations Manual