OPERATION IN BUS CONTROL MODE
For this mode of operation the RT/BC pin must be held in
the logic zero state. On power up the PUC or RESET line must
be pulsed low for a minimum of 500ns causing the chip set to
initialise and assume the halted state with the HALTED output
low. To release the terminal from the halted state, the
subsystem must drive the HALTREQ line through a low to high
transition, at which time the HALTED line will go inactive.
When the HALTED line goes inactive,the terminal will
address a four word deep Instruction Store as shown below,
using the C0 and Cl outputs.
This first instruction after a Reset is a NOOP.
C1 C0 Word
1 Receive Command
0 Transmit Command
1 Data Pointer
The instruction word specifies the operation which the
terminal is to carry out, and is formatted in the following way:
15.14. 13.12.11. 10.9.8.7. 6.5. 4.3.
Retry Retry Interrupt Bus Functlon Message
Count Condition Conditlon Select Code Code
The significance of the instruction word bits are as follows:
Code Transfer Type
000 RT to BC
001 BC to RT
010 RT to RT, data to BC subsystem
011 RT to RT, no data to BC subsystem
100 Broadcast RT to BC, non data mode commands only
101 Broadcast BC to RT
110 Broadcast RT to RT, data to BC subsystem
111 Broadcast RT to RT, no data to BC subsystem
Mode Codes without data are followed by a NOOP.
Code Terminal Function
00 Execute message code
01 Perform self test
10 Monitor bus
11 No operation (NOOP)
The Function Code (bits 4 and 3 of the Instruction Word)
specifies the required terminal mode of operation.
Execute - Code 00
With the Function code bits set to 00, the terminal will
execute the message as defined in the Message code bits
Self Test - Code 01
If the terminal has been selected to perform a Self Test
then the terminal transmitter output stages will be disabled and
the self test sequence entered. At the end of the Self Test the
transmitter stages will be re-enabled and a Report sequence
will be activated, in order to report on the success, or failure, of
the Self Test.
Passive Monitor - Code 10
If the Function code of the Instruction word is 10 the
terminal will disable the transmitter output stages, suspend
table execution and merely monitor the specified bus for valid
No Operation - Code 11
The No Operation code provides a means of introducing
delay or a wait sequence into the table operation. In selecting
this code the terminal will be forced into the Report sequence
and provide either an increment signal (INCADRN) or an
interrupt (IRQN) if the Interrupt Always flag in the Instruction
word has been selected.
00 Transmit on bus 0
01 Transmit on bus 1
Note: Bit 6 of the instruction word is tied low internally.
The required data bus on which transactions take place is
defined by bit 5. In addition to this, this bit defines the bus on
which the Transmitter Self Test operation will be conducted
and the choice of the bus for monitor purposes in Passive
Interrupt if no response
Interrupt if status bit set
Interrupt if word error
If the terminal detects one of the above conditions and the
appropriate flag is set, the the IRQ line will pulse low for 250ns.