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Chapter 6, COMPUTER OPERATION
6.3.08 Terminators
6.3.09 Delimiters
6.3.10 Status Byte, Prompts and Overload Byte
6-4
faster than the lock-in amplifier and bytes can easily be lost if the command is sent
from a program. (Note that because of the limited speed of human typing, there is no
problem in the terminal mode.) To overcome the problem an echo handshake should
be used. This works in the following way: after receiving each byte, the lock-in
amplifier sends back an echo, that is a byte which is a copy of the one that it has just
received, to indicate that it is ready to receive the next byte. Correspondingly, the
computer should not send the next byte until it has read the echo of the previous one.
The computer can compare each byte with its echo to provide a useful check on the
validity of the communications.
Where the echo is not required, it can be suppressed by negating bit 3 in the RS232
parameter byte. The default (power-up) state of this bit is for it to be asserted.
In order for communications to be successfully established between the lock-in
amplifier and the computer, it is essential that each transmission, i.e. command or
command response, is terminated in a way which is recognizable by the computer
and the lock-in amplifier as signifying the end of that transmission.
In RS232 communications, the lock-in amplifier automatically accepts either <CR>
or <CR,LF> as an input command terminator, and sends out <CR,LF> as an output
response terminator except when the noprompt bit (bit 4 in the RS232 parameter
byte) is set, in which case the terminator is <CR>. The default (power-up) state of
this bit is zero.
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 any
printing ASCII character and is common to all interfaces; it is set via the RS232
Settings menu or by the use of the DD command.
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 front panel indicators, the prompt is a
question mark character ? (ASCII 63). Otherwise the prompt is and 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
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