CF 126
6
Nov 2001
3.6.2 Command Codes
The first character of each two-character command code signifies if that command is a function command
("F"), a read operating parameter command ("R"), or a write operating parameter command ("W"). Read
and write commands can be used instead of the front panel setup menus to configure the operation of the
instrument. Function commands are used as an alternative to the front panel to gather data from the
instrument or to cause it to perform an operation.
3.6.3 Attention Character
All characters received before the "#" (ASCII code decimal 35) attention character are ignored. Therefore,
one or more instruments can share a single communications link with other "foreign instruments" (within
the physical limits of the RS-232 8 or RS-422/RS-485 interface, of course) as long as the host or the
"foreign instruments" do not use the "#" character.
When the "#" attention character is sent, the instrument enters its receive mode and will attempt to parse
out the rest of the characters before the carriage return as a possible message. If another attention
character is received in the middle of a message, the previous incomplete message is ignored. If the
instrument receives a garbled character, or if the character received has an ASCII value larger than 127,
then the entire message sent is ignored.
3.7 Response Format
An instrument will give a response over the serial communications link for every valid and invalid
command given that uses its unique two-character address. There are four types of responses used:
•
"OK", which is sent after an instrument has accepted the data presented with a write operating
parameter ("W") command, or when a function ("F") command has been completed.
•
"ERROR", which is sent when an instrument has been asked to perform an invalid command, or
when invalid information is given with a write operating parameter ("W") command.
•
"N/A", which is sent when you are requesting information from the instrument which is not
applicable to its present configuration. For example, requesting limit setup information from a DFI
1550 will cause this message to be sent.
•
a floating point number as the result of a read operating parameter ("R") or function ("F")
command.
•
other ASCII strings, such as serial numbers, dates, the front panel contents, etc.
To signal the end of a response the instrument will transmit a carriage-return (ASCII code decimal 13) if
the Auto-Linefeed function is off, or a linefeed/carriage- return (ASCII codes decimal 10, decimal 13)
when the Auto-Linefeed is on. The Auto-Linefeed status can be viewed from the front panel but can only
be changed over the serial communications link using the "W2 Write Automatic Line-feed Set- ting"
system command.
Chapter 4 RS-232 Installation Notes
4.1 Introduction
This chapter provides wiring examples and hardware information for RS-232 communications. "DTE"
(Data Terminal Equipment) will be used to refer to the personal computer, programmable controller,
terminal, data acquisition system, etc. to which the instrument is connected.
4.2 System Connector Pinout
The table below lists the pins on the System Connector used for RS-232 communication. Other pins on
the System Connector may be used for other purposes.
Table 4-1: System Connector Pins used for RS-232
Pin
Name
Function
Input/Output
Reference Pin
2
RS-232 IN
RS-232 Data In
Input
7
3
RS-232 OUT
RS-232 Data Out
Output
7
4
CTS
RS-232 Clear to Send (connected to pin
5)
N/A N/A
5
RTS
RS-232 Request to Send (connected to
N/A
N/A
