Chapter 6, COMPUTER OPERATION
6-5
response is finished and the instrument is ready for a new command. The prompt
takes one of two forms. If the command contained an error, either in syntax or by a
command parameter being out of range, or alternatively if an overload or reference
unlock is currently being detected, the prompt is a question mark character ? (ASCII
63). Otherwise the prompt is an asterisk * (ASCII 42).
These error conditions correspond to the assertion of bits 1, 2, 3, 4 or 6 in the status
byte. When the ? prompt is received by the computer, the ST command may then be
issued in order to discover which type of fault exists and to take appropriate action.
Because of the limited number of bits in the status byte, it can indicate that an
overload exists but cannot give more detail. In this case sending the N command
returns the overload byte which gives details of the location of the overload.
The prompts are a rapid way of checking on the instrument status and enable a
convenient keyboard control system to be set up simply by attaching a standard
terminal, or a simple computer-based terminal emulator, to the RS232 port. Where
the prompt is not required it can be suppressed by turning it off using the Prompt
control on the RS232 Settings Menu – see section 5.3.17
A summary of the bit assignments in the status byte and the overload byte is given in
table 6-1 below.
Bit Status Byte Overload Byte
bit 0 command complete X(1) output overload
bit 1 invalid command Y(1) output overload
bit 2 command parameter error X(2) output overload
bit 3 reference unlock Y(2) output overload
bit 4 output overload - read CH1 output overload
overload byte to determine location
bit 5 new ADC values available CH2 output overload
after trigger
bit 6 input overload CH3 output overload
bit 7 data available CH4 output overload
Table 6-1, Status and Overload Byte Bit Definitions
6.4 USB Operation
6.4.01 Introduction
The USB interface in the instrument supports operation at both Full and High Speed
settings. It provides two bulk data transfer endpoints, with endpoint 1 being used to
send commands to the instrument, and endpoint 2 being used to receive responses
from it.
Operation of the instrument via the USB is made a great deal simpler if the user
installs the relevant SIGNAL RECOVERY USB driver software. Two versions of
driver are available; the first, a bulk USB driver SRUSBXP.SYS is compatible with
several software packages supplied by SIGNAL RECOVERY, while the second,
based on National Instrument’s VISA software, allows operation both from
SIGNAL RECOVERY software and a free LabVIEW driver. The only disadvantage
of using the VISA software driver is the need to download and install the complete
VISA environment in order to use the driver.
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