Chapter 6, COMPUTER OPERATION
6-4
byte) is set, in which case the terminator is <CR>. The default (power-up) state of
this bit is zero.
6.3.08 Delimiters
Most response transmissions consist of one or two numbers followed by a response
terminator. Where the response of the lock-in amplifier consists of two numbers in
succession, they are separated by a byte called a delimiter. This delimiter can be one
of several ASCII characters and is common to all interfaces; it is set via the RS232
Settings controls or by the use of the DD command. The default value is a comma.
6.3.09 Status Byte, Prompts and Overload Byte
It is useful for a controlling program to be able to quickly determine the status of the
connected instrument. Internally, this is represented by two 8-bit values, known as
the status and overload bytes.
In RS232 communications, comparatively rapid access to the status byte is provided
by the prompt character, which is sent by the lock-in amplifier at the same time as
bit 0 (indicating command complete) becomes asserted in the status byte. This
character is sent out by the lock-in amplifier after each command response (whether
or not the response includes a transmission over the interface) to indicate that the
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 reported by the web control panel indicators, 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.
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 setting the noprompt bit, bit 4 in
the RS232 parameter byte. The default (power-up) state of this bit is zero.
Because of the limited number of bits in the status byte, it can indicate that an
overload exists but cannot give more detail. An auxiliary byte, the overload byte
returned by the N command, gives details of the location of the overload.
A summary of the bit assignments in the status byte and the overload byte is given in
table 6-1 below.
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