2.4.5ATT EIA485 Wiring
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. Figure 2-4 illustrates the typical EIA485 wiring.
The communication distance that 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 B- power line acts as a common reference tie to the Modbus master. Figure 2-5 on page 8 illustrates the EIA485 Modbus system.
| | | TB2 |
| | | TB2 |
| | Power Supply |
| 6 | | |
| 5 | +48 | |
| VDC | Common |
| |
| 4 | | |
| | RTU 8130 with 8230 expansion module |
Figure 2-4: Typical EIA485 Wiring
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