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electrically and electromagnetically noisy environment. In these cases, it is
recommended to decrease the baud rate .
5.3.5 Data Format
The PM295 supports three data formats: 7 data bits with even parity check, 8 data
bits with no parity, and 8 data bits with even parity.
In the Modbus RTU protocol, the only 8-bit data format with parity or without
parity check may be used.
5.3.6 Handshaking
Handshaking, known also as flow control, is used to accommodate transmission rate
of the Powermeter to the needs of the master computer or a printer. It may be
necessary to compensate for the Powermeter's ability to send characters faster than
they can be accepted by a modem or printed on a serial printer when the incoming
data buffer is too small. The Powermeter allows hardware and software
handshaking.
When hardware handshaking is selected, the Powermeter will not send characters to
the communications link until the DSR/CTS signal is asserted high. Whenever
DSR/CTS dropped while transmission, the Powermeter suspends sending data until
DSR/CTS is restored. Hardware handshaking applicable in all communications
modes.
NOTE
The DSR/CTS signal has an RS-232 bipolar logic level. When RS-422/485
communications is used, level conversion is necessary.
Software handshaking is applicable only in ASCII RS-232 and RS-422 full-duplex
communications modes. When a physical connection between the Powermeter and a
master or a printer is impossible or where hardware flow control is not supported,
software handshaking can be used instead. Software handshaking supports the
character flow control protocol known as XON/XOFF. The master should send an
XOFF character (ASCII DC3) when it wishes the Powermeter to pause in sending
data, and an XON character (ASCII DC1) when it wishes the Powermeter to
resume.
104
Communications Operation
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