IEEE-488 Configuration:
In the IEEE-488 mode, switches 1-5 determine the address
of
the device on the
IEEE-488 bus. The selection
is
binary from 0 to
31
with an open switch position
representing a binary 1. For example, the IEEE-488 address 7 would have
switches
1,
2,
and 3 open and switches 4 and 5 closed. When the interface is in
IEEE-488 mode, all data received
or
transmitted is echoed out the RS-232 port
at a fixed 9600 baud, no parity and 2 stop bits.
Low Level Communication Protocol:
The LEP low-level communication protocol is an efficient binary communication
protocol with access to all commands on all modules in the system. The low-level
protocol forms a message
in
five (5) parts: address, command, length, data, and
end
of
transmission. The protocol must be strictly adhered to in order to avoid
unnecessary system hangups. The low-level communication mode can be initiated
at any time by sending the byte sequence:
OxFF
Ox42.
First the device address is sent (this
is
the module bus address, not the IEEE-488
bus address). The address
is
a single byte with a range from 1
to
254 (255 is
reserved). Once the address byte is received the device is selected on the bus and
the communication must be completed according to the protocol.
If
the
communication is not completed, the system will hang.
After the device is selected with the address byte, the command is sent. The
command byte is immediately sent to the device which then prepares to either
execute the command, send data back to the host,
or
receive more data from the
host. Thecommand isa single byte from the known command list published with
each device.
If
an unrecognized command is received by the device, the
command is ignored, but this may cause problems
if
the command is interpreted
as
read command.
Following the device command, a data length byte is sent. The data length is sent
so the interface knows how many bytes to send or receive from the device and
how many bytes to send
or
receive from the host.
If
the number
of
bytes is
incorrectly sent, the protocol will become confused and the system will probably
hang. Each command has a command data length.
Some commands have no data associated with them, and a zero byte is sent after
the command to indicate that there is no data.
After the address, the command, and the data length are sent, the data follows.
If
the data is to be read back from the MAC 2000, the host computer reads bytes
from the interface until the programmed number
of
bytes is received. When data
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