Section 4.8

Orion 2/1900 Direct Data Link

4.8.1  General

The direct data link consists of a standard Orion 2 rack and plinth containing the control module etc.  The control module presents a Standard Interface to the 1900 to which it is connected via two Standard Interface cables, each having a maximum length of 100 feet.

The direct data link appears to Orion as two fast peripherals either of which will appear disengaged unless a command to READ or WRITE has been received from 1900.  The two peripherals cannot both be engaged together as the controls, situated in an Orion, share common buffers such that apparent simultaneous two way transfers in fact occur consecutively.  All commands to the direct data link are initiated by 1900, Orion then completing the link as required.  To prevent Orion having to wait up to one second before servicing a transfer request from 1900, an interrupt causes entry to the time sharer whenever one of the controls becomes "engaged".  Transfers by 1900 in modes 0 or 1 give no 'reason for entry' for the interrupt but a mode 9 transfer sets 'peripheral incident' and D6 (select) in the interrogation reply to call in Primary Input,  An advancing lockout is imposed on the area of store in Orion during a transfer.

The Direct Data Link is designed to operate at speeds up to 300 Kch/sec.

 

4.8.2  Modes

Reading and writing through the direct data link are effected by compound instructions such as:-

    140.1     0     *IL1   | Read 128 Orion words into
    142     BUFFER  128    | BUFFER onwards.

    140.21    0     *OL10  | Write 128 Orion words
    142     BUFFER  128    | from BUFFER onwards.

The modes associated with the direct data link are:-

         (   1 Read y words or to end of transfer
IL      (
         ( 13 Interrogate

         ( 21  Write y words or to end of transfer
OL    (
         ( 13  Interrogate

 

4.8.3  Data Checks

Tests are made by the Direct Data Link control on the odd parity of characters with their parity bits sent in either direction across the Orion/1900 interface.  Character parity in inserted by 1900 on transfers either way, but not checked by it.  In the case of transfers in to 1900, 1900 inserts its own parity bit for a character and returns the character with its parity bit to Orion (within the same character time) for comparison with the parity bit generated by Orion for that character.  In the case of transfers in to Orion, 1900 puts out each character with its parity bit and a test is made in the direct data link control.  Character parity failure in either direction causes data flow to stop after the current eight characters being sampled and the error condition to be signalled to Orion by means of a peripheral incident.  When the peripheral incident is sampled, Orion will signal to 1900, "P1 Status On" followed by the P1 Status condition "Data Parity Error".  If the transfer was in to 1900, the error character(s) will be within the last two 1900 words transferred.  If the transfer was in to Orion, the error character(s) will be in the direct data link control buffers.

When transferring in to 1900, a further check is made that the Orion word parity bit from the central processor to the direct data link control compares with a word parity bit formed from the eight characters put out to 1900.  Failure causes the same actions as character parity failure.

With transfers in to Orion, a parity bit for eight characters is formed in the direct data link control for transmission with a word of data to the Orion control processor.  When receiving this word, the Orion control processor checks its parity before writing it away to store.  If this check-fails, Orion comes to a complete halt, displaying an "Inoperable" condition to 1900.

 

4.8.4  Controls, Indicators and Incidents

As the direct data link requires no physical action by the operator, there are no controls or indicators other than the engineers' standard monitor panel.

Reading Incidents

Message

PARITY FAIL

 

 Cause

Parity failure detected
in character received
from 1900

 OMP Default Action

Job halted

Message printed on Flexowriter.

 

Writing Incidents

Message Cause OMP Default Action
PARITY FAIL

 

 

 

Parity failure detected
in character received
back from 1900 or in word
from Orion C.P.U. to
direct data link control

 

 Job halted

Message printed on Flexowriter
SHORT TRANSFER

 

 Transfer terminated by 1900
with Orion set to send more
words
 		 Job halted

Message printed on Flexowriter