Unit installation

7-2

TTL versus RS-232 Signal Levels:

Some devices, like HF remote base radios, require an adapter to convert their serial ports to the

signal levels used for RS-232 (-12 and +12 volts). Without that adapter, they use TTL signal levels

(0 and 5 volts). You should never connect a serial port that uses TTL level signals to one that uses

RS-232. It is possible to make the serial ports on the radio cards use TTL level signals by installing

jumper blocks on header J5, rather than installing a DS1275 in socket U8. The jumper blocks

should short pins 1 and 2 together and 3 and 4 together.

The SPI Serial Format:

There is another type of serial protocol that is totally unrelated to RS-232. It is used for controlling

some remote base interfaces, such as the Doug Hall RBI-1, RLC-ICM and BCD radio interfaces.

Instead of using a data in and data out line like RS-232, it uses two lines for output: data out and

clock out. There is no input to the controller; data is transferred only one way. Since there is a

clock line, there is no need to specify a baud rate (although there are a few timing restrictions).

The serial ports on the radio cards can also output this data format with jumper blocks on header

J5, as described in the TTL section above. The processors on the radio cards automatically switch

between SPI and TTL/RS-232 depending on what type of remote base interface they are trying to

control. These outputs are open collector and require pullup resistors (pullups are built into the

RBI-1 and RLC-ICM).