Handshaking And Echoes; Terminators - Ametek 7230 Instruction Manual

options may be used, but the no-parity option will result in slightly faster
communication.
Where the RS232 parameters of the terminal or computer are capable of being set to
any desired value, an arbitrary choice must be made. In the model 7230 the
combination set at the factory is even parity check, 7 data bits, and one stop bit
(fixed).

6.3.06 Handshaking and Echoes

A handshake is a method of ensuring that the transmitter does not send a data byte
until the receiver is ready to receive it.
In the RS232 standard there are several control lines called handshake lines (RTS,
DTR outputs and CTS, DSR, DCD inputs) in addition to the data lines (TD output
and RD input). However, these lines are not capable of implementing the
handshaking function required by the model 7230 on a byte-by-byte basis and are not
connected in the model 7230 apart from the RTS and DTR outputs, which are
constantly asserted.
Note that some computer applications require one or more of the computer's RS232
handshake lines to be asserted. If this is the case, and if the requirement cannot be
changed by the use of a software switch, the cable may be used in conjunction with a
null modem. A null modem is an adapter that connects TD on each side through to
RD on the other side, and asserts CTS, DSR, and DCD on each side when RTS and
DTR are asserted on the opposite sides.
With most modern software 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 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.

6.3.07 Terminators

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
Chapter 6, COMPUTER OPERATION
6-3