Despatch 926E Equipment Manual (C-155)Rev 2/11 - Manual (Page 35)

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Digital Communication
Digital communication is available in the following formats:

RS-232 Modbus
®
RTU

RS-485 Modbus
®
RTU
RS-232 is a common, unbalanced communication configuration. It is the type of
communication supported by the typical COM1 and COM2 hardware of a standard PC.
There are three main wires (negative transmit, negative receive, common). Some
devices use other handshaking protocol wires (RTS, CTS, etc.), but Watlow controls do
not.
RS-485 is a balanced communication configuration. This means that the transmit and
receive lines are in pairs. Typical wiring is two wire: negative transmit/receive, and
positive transmit/receive. Some instruments also have a common. The Watlow
controls do have a common.
RS-485 supports up to 32 devices without repeaters. Since there is only one pair of
wires, a control cannot listen and talk at the same time. Listen or talk state must be
selected by either handshaking hardware or signal driven handshaking. The Watlow
controls use signal driven listen/talk state enable. They are in listen state unless
addressed by a master. See the Watlow manual information on the communication
protocol for multiple devices. The RS-485 single pair wiring allows any or more than
one device to be the master.
See the Watlow EZ-Zone PM manual, wiring section EIA-232/485 Modbus RTU
Communications, for detailed instructions for installing. Additional hardware may be
required (i.e. additional cables).