Dolby Laboratories CP650 Installation Manual download pdf (Page 176)

Dolby

Model CP650 Installation Manual External Control of CP650

E-4

E.2 Control via Remotes Connector

CP650 control using single-wire communications via the remotes connector is

available in CP650 System software version 1.1.5 and later. To do so, it is

recommended that a Cat. No. 779 Remote Control or Cat. No. 771 Remote Fader be

used.

Dolby Laboratories occasionally receives requests to interface the CP650 to other

types of computers, for the purposes of remote control. While this was never intended

to be a feature of the remote port, it may be possible to command the CP650 to

change the fader level, select any one of the eight front-panel formats, and to toggle

the mute status.

The remote port interface consists of three wires. One provides 15 volts at low current

to power the remote units. The other two lines of the remote port are ground (0 volts)

and data. The ground line should be connected to the reference ground voltage of the

serial port on the computer. This is usually found on pin 7 of a serial 25-pin

D-connector, or pin 5 of a 9-pin D-connector.

The single data line presents the single biggest challenge of interfacing another

computer to the CP650. All industry-standard RS-232 ports have two data lines; one

to transmit, and one to receive. The CP650 data line works by placing the single data

line in a “high impedance” state while listening for transmitted data, and driving

voltages onto this line when it needs to send information. In other words, the single

data line is a “receiving” line for all devices, except when they need to send data, at

which point it is a “transmitting” line.

Thus, when the CP650 needs to communicate, or a remote accessory needs to

communicate with the CP650, it “takes control” of the single data line just long

enough to send its message. It then must place the line back in a high-impedance

state, and listen for a response.

Interfacing an external computer's serial port to the single data line can be tricky. The

actual requirement, as stated above, is for the serial port to “tri-state” its transmitter

when it is not sending.

If a one cannot engineer this type of circuit for the computer's serial port, an alternate

method can be tried, but it may damage the serial port. Pass the serial port's transmit

line through a series-connected resistor, and tie that line to both the serial port receive

line and the CP650's data line. If one is lucky, the serial port's transmitter (which does

not tri-state) can be overridden by the CP650's transmitter, and yet still function as a

transmitter when necessary (when the CP650 is in “listen” or high-impedance mode).

To do this, start with a high resistance (say, 100 kΩ), and reduce it until the

computer's serial port is able to both send and receive data. The exact value required

depends on the types of chips used in the serial port.

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Dolby Laboratories CP650 Installation Manual Page 176