Sensoray 417 - Manual (Page 8)

Chapter
4
Programming 4
The Model 417 is programmed by means of commands. Commands are sent from the ISAbus mas-
ter—the “host” processor—to the 417. Some commands cause responses from the 417, which are
sent from 417 to host. This chapter explains the 417-to-host communication mechanism and
describes the command set.
Programming Model
The coprocessor occupies two consecutive port addresses in ISAbus I/O space. Each port has a
unique function for read and write operations.
The base port (417 board low port address) forms the data path between 417 and host. Commands
are sent to the 417 and responses are passed back to the host through this port, which consists of
two physical hardware registers: command and data. When the host sends a command to the 417,
it is really storing a byte in the command register. When the host reads a command response from
the 417, it is reading a byte from the data register. Because the command register is write-only and
the data register read-only, they are able to share the same I/O address.
The control port (base address + 1) consists of a read-only status register and write-only control
register. As the name implies, the status register supplies coprocessor status information to the
host. The control register is used to invoke soft resets.
Status Register
The status register provides the host with information needed for status monitoring and communi-
cation handshake control. When the host reads the status register, a byte of the following form is
returned:
Table 4: Programming model
Port Address Read Write
Base + 0: Data Data Register Command Register
Base + 1: Control Status Register Control Register
Table 5: Status Register
D7 D6 D5 D4 D3 D2 D1 D0
CRMT DAV ALARM FAULT X (DISREGARD THESE BITS)