User`s manual

CHAPTER 9 BUS TUTORIAL 69
2) If a service request i8 present, the controller sets the serial poll mode. The serial
poll mode is initiated by the r controller sending the command SPE (Serial Poll
Enable).
3) The controller sequentially polls those devices that may a have requested service.
Each polled device responds with the status byte. The controller then checks the bit
6 (weight 64) of the byte to see if service was requested by this device.
4) For each device that has requested service, the controller takes appropriate action.
5) When all devices have been polled, the controller terminates the serial poll mode by
issuing the command SPD (Serial Poll Disable).
The full sequence of operations is not necessary in all cases. If the reason for a request
is simple and the controller knows the action, serial polling is not necessary. For
software convenience, the controller sends SPD just after the status byte is read by
the computer.
9.6. Parallel Polling
Parallel polling permits the status of up to eight devices on the bus to be checked
simultaneously. Each device is assigned a data line (DIO1 to DI08) during parallel poll
configure. When the controller conducts a parallel poll (ATN and EOI set low at same
time), the device sets the assigned data line low or high to indicate if it requires service.
If more than eight devices are used with their parallel poll capability, some of them can
share one data line. Very few instruments have the capability to respond to a parallel
poll. Therefore, you can neglect the parallel poll now and only study it when it
becomes necessary.
9.7. Code Summary
A code assignment is shown in the table on the next page. These assignments apply
only in command mode. In data mode there are no specific code assignments and data
strings are recognized by the devices which receive the data.
The set of codes labeled “Primary Command Group” are the codes commonly used to
communicate on the bus. The “Secondary Command Group” is used when addressing
extended listeners and talkers, or enabling the Parallel Poll Mode (PPE).