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14.6 Teco-Westinghouse ASDs
The gateway can act as a Modbus RTU protocol master via its RS232 and RS-
485 ports, which allows connection to Modbus-capable Teco-Westinghouse
ASDs. While it is possible to connect to any Teco-Westinghouse ASD that
supports Modbus (either RS232 or RS485), this section will discuss in particular
some important considerations to make note of when connecting to, and
interacting with, the PA7300 (with installed PA-M card) and MA7200 ASDs via
RS485.
14.6.1 MA7200
•
Connect the gateway's RS485 port "A" terminal to the drive's "S+" terminal,
the gateway's RS485 port "B" terminal to the drive's "S-" terminal, and the
gateway's RS485 port "SG" terminal to the drive's "GND" terminal.
•
Although the latest MA7200 Modbus manual (dated 2001/06/05) indicates
that the maximum baud rate (set via parameter Sn-37) is 9600 baud, all
newer drives actually support 19.2kbaud (Sn-37 = 4).
•
When drive parameter Sn-38 (parity) is set to 0 (no parity), the gateway's
port configuration can be set to either "none / 1 stop bit" or "none / 2 stop
bits" (either setting will work).
•
Note that the default value of drive parameter Cn-27 (time-out check) is
1.0s. Because the minimum timeout time that the gateway's port can be
configured for is 1s, to avoid nuisance timeout faults on the drive it is
recommended to either set parameter Cn-27 to 0.0s (if timeout checking is
not needed), or to a value of 2.0s or more (if timeout checking is desired).
•
If run/stop etc. commands are to come from network, set drive parameter
Sn-04 to 2.
•
If frequency reference is to come from network, set drive parameter Sn-05
to 2.
•
Note that the holding register "address" column provided in the MA7200
Modbus manual contains the Modbus "addressed as" (on-the-wire) values,
which are always 1 less than the Modbus "known as" values (which the
gateway's configuration requires). Additionally, this "address" column
indicates the holding register numbers in hexadecimal, which is atypical for
Modbus decimal-based register assignments. Therefore, to calculate the
"register" assignment that must be programmed into the gateway, first
convert the documented "address" to a decimal value, and then add 1. For
example, the MA7200's "output frequency" register is documented to exist
at register 0025H. Converting this number to decimal and adding 1
therefore results in a "known as" register value of 38, which can then be
entered in the Modbus master "register" assignment of a point on the
gateway.
•
The MA7200 drops Modbus packets frequently when it is in PRG mode.
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