Wavelength Electronics LFI-3751, User Manual

Page: 80 / 132

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The host computer sends a Command Packet to the instrument and the
temperature controller returns a Reponse Packet to the host computer. A
Response Packet always returns after each Command Packet is sent. The
[HALT PROCESSOR] command is the only exception. The processor is
immediately halted and does not return a Reponse Packet .
•
All characters in the Command and Response Packets are ASCII. The
character fi elds are decimal [‘0’ to ‘9’] except for the Frame Check Sum
(FCS) which is represented in HEX [‘0’ to ‘9’ or ‘A’ to ‘F’].
•
The characters within the packets follow a rigid structure. The instrument’s
parser expects 17 characters in a Command Packet . Extra Command
Packet characters will be ignored until a new start character is received.
A Response Packet always contains 21 characters.
•
READ commands (Command Type 1) do not use the data fi eld. The data
fi eld must still contain a number between -999.999 and +999.999.
•
There are no restrictions on how fast the host computer sends characters
to the instrument. The packets can be sent at one character per second
or using the full 19200 baud capacity. If the host computer can’t buffer the
instrument’s
Response Packets at full speed, confi gure the host computer’s
RS-232 port for XON/XOFF fl ow control.
•
When a value is written to the instrument in a Command Packet , the
corresponding Response Packet will return the instrument’s interpretation
of the value. For example, if the data fi eld for the [SET T] Command
Packet is +120.263, then the Response Packet will 120.300
in its data fi eld.
•
The instrument’s measurements are updated about every 600 milliseconds
with an additional delay of about 750 milliseconds every ten minutes
when the unit automatically zeros the instrument’s measurement
A/D. The 19.2 k BAUD rate allows Measurement Reads about every
19.8 milliseconds. The instrument returns the most recent value
immediately so as not to tie up the interface in a multi-packet, multi-
instrument environment. If a new measurement is required for each
Command Packet , program in a 800 millisecond delay between Command
Packets .
•
No Response Packet is returned from any unit unless it at least receives a
valid Start Character, a matching Unit Type and Unit Number.
Chapter 4 - Remote Interface Reference
RS-232 Command and Response Packet Formats
80
RS-232 Command and Response Packet Formats