2920 Float & Tape Transmitter MODBUSAppendix A — MODBUS Implementation
158 Installation and Operations Manual
Hardware Implementation
The 2920 FTT 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 2920 FTT 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 2920 FTTs. The recommended wire for EIA485 systems is 18-gauge
or larger, shielded, twisted pair. The shield should be earth grounded at the MODBUS master
(control system or computer end). The shield at the 2920 FTT should be open. The 2920 FTT-
power line acts as a common reference tie to the MODBUS master.
Termination resistors of 100 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. These resistors are included in the 2920 FTT
and can be included in the circuit using the procedure described in “Setting the Bias Current”.
Integer Register Map
Register Number Type Description
0 Scaled Level (MODBUS > Level)
1 Scaled Temperature (MODBUS > Temp)
2 Reserved
3 Scaled Unit Temperature (Encoder Data > Internal Temp)
4 Integer Number of Resets (Encoder Data > NumberResets)
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