14
COMPUTER CONTROL
Serial Interface Electrical Connections & Cabling: The 9-pin Subminiature D (male) connector on the PMX84 rear panel provides the
RS-232 compatible serial interface signals used for computer control. The PMX84 transmits serial data on pin 3 (TxD) and receives serial
data on pin 2 (RxD). The serial interface ground is on Pin 5. The DTR & RTS signals are connected to the +12 Volt power supply (each
through its own resistor) and are always asserted when the PMX84 power is on. Most IBM compatible PCs use either 25-pin or 9-pin
(male) connectors for their serial ports. The following table summarizes the pin assignments for the PMX84 serial interface, and for the
standard IBM compatible 9-pin and 25-pin serial ports.
SIGNAL NAME
DIRECTION
PMX84 9-PIN
IBM-PC 9-PIN
IBM-PC 25-PIN
CD (carrier detect)
input
n/a
pin 1
pin 8
RxD (receive data)
input
pin 2
pin 2
pin 3
TxD (transmit data)
output
pin 3
pin 3
pin 2
DTR (data terminal ready)
output
pin 4
pin 4
pin 20
signal ground
n/a
pin 5
pin 5
pin 7
DSR (data set ready)
input
n/a
pin 6
pin 6
RTS (request to send)
output
pin 7
pin 7
pin 4
CTS (clear to send)
input
n/a
pin 8
pin 5
RI (ring indicator)
input
n/a
pin 9
pin 22
The PMX84 only requires receive data (pin 2), transmit data (pin 3), and signal ground (pin 5) to be connected for successful data
communications. However, the PC may require that signals be present on the data set ready, clear to send, or carrier detect inputs, as
well as the receive data, transmit data, and signal ground pins. The diagrams on the following page show cables for interfacing to a PC
with either a 9-pin or a 25-pin serial port connector. In most cases, one or the other of these cables will work. However, success or failure
depends entirely on the actual computer hardware and software being used. When trying to solve an interfacing problem, the most
important thing to remember is that an output of one device should connect to one or more inputs of the other device, and that two outputs
should never be connected together. Also, keep in mind that the RS-232 specification calls for the cable length to be no greater than 50
feet (although it is not unusual to be able to operate over distances of 150 to 250 feet), and the connectors must be of the appropriate
gender (male or female) to mate properly. For best results, a shielded cable should be used, with the shield connected to signal ground.
Since the PMX84 serial interface ground is also tied (indirectly) to the analog signal ground, undesirable ground loops may occur when the
PMX84 is connected to a PC (if the system grounding is not carefully designed). For best performance, the PC ground and the chassis
ground of the PMX84 should be at the same potential, and the PC should get AC power from the same source as the PMX84 (and any
other audio equipment which is connected to the PMX84).
Serial Interface Data Communications Parameters: The PMX84 communicates through the serial interface at a rate of 9600 bits per
second, with 8 data bits, 1 stop bit, and no parity. The PMX84 utilizes a subset of the standard 7-bit ASCII character set. The eighth data
bit of each character (the most significant bit) should always be 0. The computer should not echo the characters it receives. The computer
should not be set for either hardware (DTR) or software (XON/XOFF) flow control. The baud rate may be changed to 2400 bits per second
by means an internal jumper strap (J31). Jumper strap J31 is located on the right front corner of the lower circuit board, directly in front of
the microprocessor (U36). To select 2400 baud rate: 1) Disconnect power from the PMX84. 2) Remove top panel. 3) Using needle-
nose pliers, lift jumper strap J31 and move it over one pin (to the left). 4) Replace top panel & turn power on.
