MA28139 datasheet(21/34 Pages) DYNEX

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MA28139

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THE ESA ON-BOARD DATA HANDLING

(OBDH) BUS

The dual redundant OBDH bus is connected to the OBT

bus interface via an input descriminator and an output bridge

driver circuit. These convert between the bipolar LITTON code

and the standard CMOS inputs and outputs of the IC.

The OBDH bus is divided into three parts:

A. INTERROGATION BUS (I BUS)

This bus is used to transfer data from the CT to the

RTs, as commands of 32 bit words, each bit being modulated

according to the Litton scheme shown in Figure 18. Each

Interrogation (or command) “slot” comprises 3 Sync bits, 3 or 4

BroadCast Pulses, 5 or 6 Terminal Address bits, 4 Destination

Address bits, 16 Data bits and a Parity bit.

B. RESPONSE BUS (R BUS)

This bus is used to send data from the RTs to the CT, (can

be used by RTs to receive data). Each response word

comprises the 4 Destination Address bits sent in the

corresponding Interrogation, either 8 or 16 Data bits from the

user (8 bits unless a 16 bit acquisition was requested and 8

zeros if no response data is required) and a single Stop bit

(used to ensure data is fully clocked through bus modems and

0 by convention).

C. BLOCK TRANSFER BUS (BT BUS)

Used to transfer blocks of data between the CT and RTs, in

either direction, as a contiguous block or stream of data bits.

Figure 18: Litton Coded Data

logical

"1"

logical

"0"

invalid

logic

"1"

invalid

logic

"0"

high -

low -

0v -

high -

0v -

high -

low -

0v -

low -

0v -

invalid

logic

"1"

high -

0v -

logical

"1"

invalid

logic

"0"

low -

Synchronisation Pattern

FASTER OBDH/DBI COMPATIBLE NETWORKS

With analog components the OBT can interface any

equipment to the specified ESA OBDH bus at the nominal data

rate of 0.5 Mbps. Contract 5352 proved that analog

components limit OBDH data rate to 2 Mbps maximum. But

OBTs work to over 5 Mbps (10MHz with 2 clocks/bit Litton

coded). OBTs may be directly networked via digital bus

drivers/receivers, (eliminating analog components) using

Litton coded 4 wire (R2/DS1 and R4/DS2) busses (see OBDH

application note 1). MSS made a 3 metre optical OBDH

network for the Pegasus ion source.

DBIs may be directly connected but will not be Litton coded

with Parity on every bit, or exhibit modulation and

demodulation delays.

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