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Operating Instructions
Note that in block mode transfers, since the 8901A is initiating
the CAMAC cycles, if the GPIB controller terminates the trans
fer before Q=0, one additional cycle will be executed and the
data will be left in the 8901A's registers. To access this data it
is necessary to send the CAMAC command F(0), A(0), and
N(24) (see next section) and then read out the data word. At
the end of the block transfer, the 8901A is set to the corre
sponding normal transfer mode.
Table 1 - Transfer Mode Commands
Normal
Transfer
Block
Read
High Speed
Block Read
8-bit read
16-bit read
24-bit read
121 (79)
105 (69)
122 (7A)
106 (6A)
124 (7C)
108 (6C)
Single byte commands - values specified in decimal
with hex value given in ()
Service Request Response
Commands
A service request (SRQ) is a mechanism by which a GPIB com
patible instrument can tell a computer that a particular condition
exists, without the computer having to read a byte of informa
tion. There are three conditions occurring on the CAMAC bus
which can cause the Model 8901A to issue a service request to
the GPIB controller: When a LAM is set, and when Q and/or
X indicate that invalid data/commands occurred during a trans
fer. Table 2 below lists the commands to send to the 890lA to
set it up to generate a SRQ on the proper condition (s).
The GPIB controller must perform a serial poll of all devices
after receiving a service request. When the 8901A is polled, it
sends up to 5 bytes of information to the GPIB controller. The
first byte is a status byte which contains the current state of X
and Q in bits 1 (LSB) and 2 respectively and bit 7 indicating
whether or not the currently addressed 8901A was the device
which requested service. If bit 7 is equal to one, the 890lA
generated the request. The next four bytes indicate the state of
the LAM lines. They are encoded as follows:
LAM for slots 1 through 6 are in byte 2 (LAM 1 is LSB)
LAM for slots 7 through 12 are in byte 3
LAM for slots 13 through 18 are in byte 4
LAM for slots 19 through 23 are in byte 5
10
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