The terminal may be configured into a Passive Monitor (or
Bus Monitor) merely by selecting the appropriate Function
Code of the Instruction word. By doing this the terminal will not
take part in any further Instruction execution but instead will
monitor the selected bus for data transmissions.
INTERRUPT / RETRY CAPABILITY
The interrupt facility provides a means of more direct
subsystem interaction in the event of a failure. Similar flags are
required to be set in the Instruction word before a selectable
interrupt may be generated. This form of interrupt also includes
an Interrupt Always flag whose application may be used to
determine subsystem/system timing requirements.
It should be noted that an interrupt may also be generated
by the error checking procedures of the terminal which verify
aspects of the Instruction word and associated Receive/
Transmit command words.
The terminal has certain in-built functions which permit the
terminal to retrieve situations which would normally cause a
greater degree of subsystem intervention. This is achieved by
having an automatic retry facility in-built to the terminal which
is selectable from the Instruction word. In this case both the
condition and number of attempts for which the terminal must
try may be specified. After completion of the required number
of attempts, terminal operation may be halted with the
possibility of an interrupt generated also.
STANDBY BUS CONTROLLER
The terminal provides a number of signals to the
subsystem for message addressing and execution. Two
address lines are provided (CO, Cl) plus a signal to increment
an external counter (INCADRN). This, together with the on-
chip sequencing, error checking, etc., enables a standby bus
controller, using a fixed table of messages, to be realised in
few devices as shown in Figure 2. It is therefore possible to
attain a standby BC on a single 6 x 4 PCB card.
Figure 2: Standby Bus Controller