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CHAPTER 5:
Computer Interface Operation
Model 648 Electromagnet Power Supply
The *OPC? query has no interaction with bit 0 (OPC) of the Standard Event Status Reg-
ister. If the *OPC? query is sent at the end of a command sequence, the bus will be
held until the instrument completes the operation that was initiated by the com-
mand sequence. Additional commands (except *RST) should not be sent until the
operation is complete as erratic operation will occur. Once the sequence is complete a
1 will be placed in the output buffer. This function is typically used to signal a com-
pleted operation without monitoring the SRQ. It is also used when it is important to
prevent any additional communication on the bus during a pending operation.
5.3 Serial
Interface
Overview
The Model 648 USB interface provides a convenient way to connect to most modern
computers, as a USB interface is provided on nearly all new PCs as of the writing of
this manual. The USB interface is implemented as a virtual serial com port connec-
tion. This implementation provides a simple migration path for modifying existing
RS-232 based remote interface software. It also provides a simpler means of commu-
nicating than a standard USB implementation.
5.3.1 Physical
Connection
The Model 648 has a B-type USB connector on the rear panel. This is the standard
connector used on USB peripheral devices, and it allows the common USB A-type to
B-type cable to be used to connect the Model 648 to a host PC. The pin assignments
for A-type and B-type connectors are shown in section 6.11.5. The maximum length
of a USB cable, as defined by the USB 2.0 standard, is 5 m (16.4 ft). This length can be
extended using USB hubs every 5 m (16.4 ft) up to five times, for a maximum total
length of 30 m (98.4 ft).
5.3.2 Hardware
Support
The USB interface emulates an RS-232 serial port at a fixed 57,600 baud rate, but
with the physical connections of a USB. This programming interface requires a cer-
tain configuration to communicate properly with the Model 648. The proper configu-
ration parameters are listed in TABLE 5-5.
Host com port
configuration
Baud rate
57,600
Data bits
7
Start bits
1
Stop bits
1
Parity
Odd
Flow control
None
Handshaking
None
The USB hardware connection uses the full speed (12,000,000 bits/sec) profile of the
USB 2.0 standard; however, since the interface uses a virtual serial com port at a fixed
data rate, the data throughput is still limited to a baud rate of 57,600 bits/s.
5.3.3 Installing the USB
Driver
The Model 648 USB driver has been made available through Windows® Update. This
is the recommended method for installing the driver, as it will ensure that you always
have the latest version of the driver installed. If you are unable to install the driver
from Windows® Update, refer to section 5.3.3.3 to install the driver from the web or
from the disc provided with the Model 648.
These procedures assume that you are logged into a user account that has adminis-
trator privileges.
TABLE 5-5
