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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
changed by the use of a software switch, the RS232 null modem cable that is used
must connect TD on each side through to RD on the other side, and assert CTS,
DSR, and DCD on each side when RTS and DTR are asserted on the opposite side,
in addition to connecting the transmit data, receive data, and logic ground signals.
In many cases there is no need to assert any RS232 handshake lines and a simple
three-wire connection can be used. The actual handshake function is then performed
by means of bytes transmitted over the interface.
The more critical handshake is the one controlling the transfer of a command from
the computer to the lock-in amplifier, because the computer typically operates much
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 when using a terminal emulator program.) To overcome the problem an
echo handshake must 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.
Although character echo can be disabled via the RS232 Settings Menu (see section
5.3.17), this is not recommended.
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
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