VersaPro Temperature Controller
Page 20
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All rights to copy, reproduce and transmit are reserved
8
Serial Interface
The VersaPro has a single RS-485 half duplex (two wire) communications port. This port can be configured
for either the Marathon protocol of Modbus RTU protocol. Baud rates and parities are selectable. The
Modbus protocol only uses a parity of none. See the section on communications for details on both of these
protocols.
The Versapro can be connected to networks with up to 128 similar devices. The differential transceiver used
in the Versapro meets or exceeds the TIA/EIA-485 and ISO/IEC 8482:1993(E) standards.
Connections for the serial interface should be connected to the following terminals:
TB-B 13
RTX +
TB-B 14
RTX –
All connections to any RS-485 bus should be made with shielded twisted pair wires using a low capacitance
cable specified for RS-485 multi-drop connections. The shield should be connected to ground on one end of
the wire run. Shield continuity should be maintained between wire segments. Each end of the network
should be terminated with a resistor that is close to the impedance of the cable. 100 to 120 ohms are typical
values. All connections to multiple instruments should be made in a daisy-chain fashion, from one instrument
to the next. A star network connection should never be used. A repeater should be considered for cable
distances beyond 3900 feet (1.2Km). Any network that is run between buildings should use repeaters with
optical fiber connections between buildings to avoid any noise spikes generated by lightning strikes.