Choice Of Baud Rate; Choice Of Number Of Data Bits; Choice Of Parity Check Option - Ametek 7230 Instruction Manual

Chapter 6, COMPUTER OPERATION

6.3.03 Choice of Baud Rate

6.3.04 Choice of Number of Data Bits

6.3.05 Choice of Parity Check Option

6-2
with a null modem or, alternatively, may be made up from low-cost general-purpose
cable. The pinout of the RS232 connectors is shown in appendix B and cable
diagrams suitable for coupling the instrument to a computer are shown in
appendix C.
The main advantages of the RS232 interface are:
1) It communicates via a serial port which is either already fitted or can be added at
low cost via an adaptor to any computer, using leads and connectors which are
available from suppliers of computer accessories or can be constructed at
minimal cost in the user's workshop.
2) It requires no more software support than is normally supplied with the
computer, for example Windows HyperTerminal.
A single RS232 transmission consists of a start bit followed by 7 or 8 data bits, an
optional parity bit, and 1 stop bit. The rate of data transfer depends on the number of
bits per second sent over the interface, usually called the baud rate. In the model
7230 the baud rate can be set to a range of different values up to 38,400,
corresponding to a minimum time of less than 0.25 ms for a single character.
Mainly for historical reasons, there are a very large number of different ways in
which RS232 communications can be implemented. Apart from the baud rate
options, there are choices of data word length (7 or 8 bits), parity check operation
(even, odd or none), and number of stop bits (1 or 2). With the exception of the
number of stop bits, which is fixed at 1, these settings may be adjusted using the
RS232 Settings controls, discussed in chapter 5. They may also be adjusted by means
of the RS command.
NOTE: In order to achieve satisfactory operation, the RS232 settings must be set
to exactly the same values in the terminal or computer as in the lock-in amplifier.
Where the lock-in amplifier is connected to a terminal or to a computer
implementing an echo handshake, the highest available baud rate of 38,400 is
normally used if, as is usually the case, this rate is supported by the terminal or
computer. Lower baud rates may be used in order to achieve compatibility with older
equipment or where there is some special reason for reducing the communication
rate.
For most transmissions, the model 7230 lock-in amplifier uses the standard ASCII
character set, containing 127 characters represented by 7-bit binary words. If an 8-bit
data word is selected, the most significant bit is set to zero on output from the lock-in
amplifier and ignored on input.
The binary dump commands, used for the fastest data transfer rates from the internal
curve buffers, require 8 bit transmission.
Parity checks are not required at the baud rates available in the model 7230 with
typical cable lengths of up to a few meters. Therefore no software is provided in the
model 7230 for dealing with parity errors. Where long cables are in use, it may be
advisable to make use of a lower baud rate. The result is that any of the parity check