Chapter 3 Front-Panel Operation
RS-232 Interface Configuration
54
DB-25 Serial Connection
If your computer or terminal has a 25-pin serial
port with a m ale conn ec tor, u se the nul l-m ode m cable and 25-pin adapter
included with the HP 34398A Cab le Kit. The cable and adapter pin diagram
are shown below.
DTR / DSR Handshake Protocol
The power supply is configured as a DTE (
Data T erminal Equipment
) device
and uses the DTR (
Data T erminal Ready
) and DSR (
Data Set Ready
) lines of
the RS-232 interface to handshake. The power supply uses the DTR line to send
a hold-off signal. The DTR line must be TRUE before the power supply will
accept data from the interface. When the power supply sets the DTR li ne
FA LSE , the data must cease within 10 characters.
To disable the DTR/DSR handshake, do not connect the DTR line and tie the
DSR line to logic TRUE. If yo u disable the DTR/DSR handshake, also select a
slower baud rate to ensure that the data is transmi tted correctly.
The pow er supp ly se ts the DTR li ne FALSE in the foll owin g two cases :
1
W hen the power supply’s input buffer is full (when approximately 100
characters have been received), it sets the DTR li ne FAL SE (pi n 4 on the RS-
232 connector). When enough characters have been removed to m ake space
in the input buffer, the power supply sets the DTR li ne TRUE , unless the
second case (see next) preven ts this.
2
W hen the power supply wants to “talk” over the interface (which means that
it has processed a query) and has received a
<new line>
message terminator,
it will set the DTR line FALSE. This implies that once a query has been sent to
the power supply, the bus controller sho uld read the response before
attempting to send more data. It also means that a
<new line>
must terminate
the command string. After the response has been output, the power supply sets
the DTR line TR UE again, unl ess the first case (see above) prevents this.
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