Measurement Computing Personal488 rev.3.0 For DOS & Windows 3.Xi - Manual (Page 146)

II. SOFTWARE GUIDES - 8. Driver488/DRV 8P. Command Descriptions
Personal488 User’s Manual, Rev. 3.0 II-131
•
Not Data Accepted ( NDAC ): Employed by an IEEE 488 interface, this line is also controlled by
the Listeners, and indicates to the Talker that each device addressed to listen has accepted the
information. Each device releases
NDAC at its own rate, but NDAC does not do so until the slowest
Listener has accepted the data byte. This type of handshaking permits multiple devices to receive
data from a single data transmitter on the bus. All active receiving devices will participate in the
data handshaking on a byte-by-byte basis and operate the
NDAC and NRFD lines in a “wired-or”
scheme so that the slowest active device will determine the rate at which the data transfers take
place. In other words, data transfers are asynchronous and occur at the rate of the slowest
participating device.
•
Clear To Send ( CTS ): Employed by a serial interface, this line is specified to assist half-duplex
communication equipment in transmitting and receiving data. When ready to receive, the receiver
asserts its
CTS line, allowing transmission to begin.
•
Data Carrier Detect ( DCD ): Employed by a serial interface, this line is asserted by the modem or
other data communication equipment (DCE) to indicate that it has established a communication
link with the modem or DCE at the other end of the communication link (e.g., phone line). It must
be asserted for the terminal to go on-line and receive data.
•
Data Set Ready ( DSR ): Employed by a serial interface, this line is specified to indicate the
presence and readiness of data terminal and data communication equipment (DCE). The DSR is
asserted by the modem or other DCE to allow the terminal to go on-line and receive data.
Data Transfer Lines
To transfer information between devices on the bus, eight lines ( DIO1 through DIO8 ) are used by the
IEEE 488 interface. As previously discussed, when ATN is unasserted, the information contained on
the data lines is to be interpreted as data for the Active Listeners. However, when ATN is asserted, that
information is to be interpreted as a bus (multiline) command.
Bus Command Groups
Bus commands are bytes sent by the Active Controller over the data bus with Attention ( ATN ) asserted.
These commands are sent to all devices and are divided into the following 5 groups:
•
Addressed Command Group ( ACG ): These commands affect only those devices which have
previously been addressed to be a Listener. There are 5 bus line addressed commands: GET , GTL ,
PPD , SDC and TCT .
•
Universal Command Group ( UCG ): These commands cause every instrument on the bus to carry
out the bus function specified (if the instrument is capable of it). There are 5 bus line universal
commands:
DCL , LLO , PPU , SPD , and SPE .
•
Listen Address Group ( LAG ): These commands address to Listen specified bus devices. The
addressed device then becomes a Listener. There are 31 ( 0 to 30 ) listen addresses associated with
this group. The 3 most significant bits of the data bus are set to 001 while the 5 least significant
bits are the address of the device being told to Listen. The last command in this group is UNL .
•
Talk Address Group (TAG): These commands address to Talk specified bus devices. The
addressed device then becomes a Talker. There are 31 ( 0 to 30 ) talk addresses associated with this
group. The 3 most significant bits of the data bus are set to 010 while the 5 least significant bits
are the address of the device being told to Talk. The last command in this group is UNT .
•
Secondary Command Group ( SCG ): These commands are used to specify a subaddress or
subfunction within a given bus device. There are 32 ( 0 to 31 ) possible secondary commands used
to specify a subaddress of subfunction within a given bus device. They are also used in the
Parallel Poll Configure (
PPC )sequence.
•
Three bus commands not found in the above groups are: MLA , MTA , and PPC .
All of the IEEE 488 bus commands are further described individually, as follows: