Valhalla Scientific 4300C, Operation Manual

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Detailed Descriptions of Bus Operations
The five management wires are described as follows:
ATN : Asserted by the controller whenever an address or a command is present on the bus.
EOI : May be asserted by the controller or any talker. With ATN true, EOI indicates that the controller
is polling devices. With ATN false, EOI is asserted by the talker to indicate the end of data.
SRQ : May be asserted by any device. This indicates that the device requires attention (e.g., a fault has
occurred). Normally, the controller will respond by polling to determine which device requires
service.
IFC : May be asserted only by the controller. This line initializes the bus to a reset state.
REN : May be asserted only by the controller. This signal places the addressed device into the remote
mode.
The three handshake wires are described as follows:
DAV : May be asserted by any talker; indicates that a valid data byte is present on the data wires.
NRFD : May be asserted by any listener. Indicates that the listener is not ready to receive data.
NDAC : May be asserted by any listener. Indicates that the listener has not yet finished reading the data
byte.
The transfer of data on the bus is controlled by these three handshake wires. It is important to note that
the drivers for the handshake wires are all connected for wired-on operation. That is, if any of the devices
on the bus asserts a handshake line, it will remain true. There must be a complete consensus among the
devices for any handshake wire to be high (false).
The talker first waits for all devices to be ready to accept data (checks that NRFD is false) then puts one
byte of data on the bus and asserts DAV. It waits for all devices to indicate that the data has been accepted
(that is, NDAC to become false) before starting to transfer the next byte of data. This handshake protocol
assures that data on the bus is transferred at the speed of the slowest device on the bus.
Data is sent in 8-bit bytes on the DIO wires, usually (as in the 4300C) using the ISO-7 standard ASCII
characters. Table 8 lists each ASCII character and the bus messages applicable to each.