Service manual

Backplane (SHN857-115)
CARD DESCRIPTION
Audio connectors
Audio modules plug into 96 way DIN connectors J1 to J12. The top half of the connectors
handles analogue audio signals or AES audio signals, the bottom half of the connector
handles the digital audio and clocks to the digital control module.
A pair of boards makes up one audio module and the 16 signals to the two boards are split
between them as follows. Looking from the front the left hand board of a pair handles
channels 1, 4, 5, 7, 9, 11, 14 and 15. The right hand board handles 2, 3, 6, 8, 10, 12, 13
and 16. If a fault is discovered in a module the channel number will therefore determine
which of the two boards the fault is on.
The audio signals comprise of three connections: H, C and G for ground. These signals are
grouped on a single row A, B and C of the 96 way connector. The 8 triples are interspersed
with the analogue power rails at the top of the connector.
Below on A20, B20, C20 and A21 are the four ID lines which tell the board its address or
position in the rack. On J1 they are all low and this is read as 0 which is then translated as
board 1 by adding 1. On J2 ID0 is connected to +5V which is translated as board 2. This is
used to map modules to MADI channels and to allow the desk and digital control module to
be able to address the boards individually with control messages.
Rows 23 to 26 of the 96 way DIN connectors are the digital audio signals to and from the
digital control module. There are four signals IN to the rack and four signals OUT to the
audio board. Each signal carries a pair of digital audio channels so forming 8 digital out
and 8 digital input lines for each board and therefore 16 in and out for each module. All are
standard TTL levels.
Row 29 carries the balanced RS485 control lines to all the audio modules. Each of the twelve
boards will respond to messages from the master module over this link. The digital control
module continuously polls the modules to check if any have been removed or replaced. It
also checks for switch press messages from the module front panels and transmits control
messages to the audio circuitry on the audio modules. The link is an asynchronous pair of
5V logic signals at 19.2kbaud which are in antiphase to each other.
A30, A31 and C30 carry the three clocks used by the audio modules for synchronization to
the rack. A30 carries wordclock of (48kHz at a 48kHz system rate), C30 carries a bit clock
(BCLK or SCLK) of 64 times word clock (3.072MHz at 48kHz sample rate) and A31 carries a
master clock (MCLK) of 256 times word clock (12.288MHz at 48kHz sample rate). All are
standard TTL levels.
Digital Control Module connectors
The digital audio and clock signals for the first 6 boards drop down to J15, the signals for the
last 6 boards are passed to the digital control module by J16.
J16 also carries the following signals from the digital control module to J68 which is the
connector for the digital I/O card.
Digital Service Manual
Issue 3 Page 36