5 CIRCUIT DESCRIPTION AND SCHEMATIC DIAGRAMS T2130
PAGE 5-12
As the level on the SP-Bus approximately matches the RS232C standard, it is possible to connect an
RS232C to the SP-Bus. The cable terminations (jumper P04) must be removed when the external
RD232C is to transmit on the SP-Bus.
SERIAL DEVICE BUS DRIVER & SELECTOR
The serial device bus consists of:
- clock
- Data Out
- Data In
- several select lines
- special select for the A/D converter
The serial bus occupies 6 bits of port 6 on the microprocessor. Bit 0, bit 1, and bit 2 are connected to the
serial device selector, bit 3 is the special select for the A/D converter, bit 6 is the clock, and bit 7 is data.
The serial device selector is a 3 to 8 line decoder 74HC237 (U07). The 3 input lines give 8 selects on the
output, one selected at a time. The following codes are used:
0 - none is selected
1 - EEPROM U11
2 - EEPROM U12
3 - shift register U10, filter module driver
4 - shift register U14, tune motor control
5 - AT2110 driver
6 - not used
7 - A/D converter, reference voltage switch
The serial device selector is supplied by +5VB.
The serial device bus driver consists of 3 lines:
- clock, port 6, bit 6
- Data Out, port 6, bit 7
- Data In
In order to have a safe communication, the clock and Data Out are buffered. The buffers are 74HC08
(U08/3 and U08/4) supplied by +5VB.
The Data In line is connected direct to Port 6, bit 7. R04 ensures a definite impedance on the Data In line.
To inhibit an unknown current sink in the serial device bus when the supply is failing, the buffers are gated
with PFO signal from the battery low detector (U03, pin 5).
EEPROMS
U11 and U12 EEPROMS are supplied by the back-up voltage +5VB. They are connected to the serial
device bus direct to the microprocessor as the serial device bus closes down when the +18V is failing.
U11 and U12 EEPROMS remember the data for the fast retune of the AT2110.
By use of service programmes it is possible to delete these retune data.
9315
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