M718 — Talkback Address Module

M718 TALKBACK ADDRESS MODULE

Logic Level: HC Logic Levels with a 5 volt supply.

Control Inputs: Logic 0 - less than 1.1 volts and greater than 0 volts Logic 1 - greater than 3 volts and less than 24 volts

Control Outputs: Open collector busy tally - 24 volts at 30 mA. Mute & Talkback relays- 30 volts at 100 mA. Talkback Address - Standard HC logic.

Power Requirements: +10 to +25 volts dc, unregulated, 70 mA.

Circuit Protection: 0.5 Amp fuse, current limiting regulator and over voltage protecting Zener diode.

Ambient Temperature: 0 to 50 degrees Celsius.

Dimensions: 0.7" wide x 4.40" high x 11" deep.

Weight: 180 grams (6.4 ounces).

GENERAL

M718 TALKBACK ADDRESS MODULE

The M718 module generates a talkback address for consoles that require a 4 bit binary address code. The module is capable of multiplexing 16 external keys and tally LEDs for address selection.

A separate talkback input enables the output of the four bit binary address. Three additional inputs are provided to send three corresponding preset address remotely. Three binary coded rotary switches on the front edge of the card provide selection of the preset address. The module provides two form C relay contacts that are activated when one of the talkback or remote address inputs are activated and four form C relay contacts that are activated when one of the talkback or remote address or Mic On inputs is activated.

One control input is provided to control an open collector output which may be used for tally LEDs that cannot be driven by standard logic gates.

The M718 is self contained in that it has its own switching regulator that can run off +24V unregulated . Inputs

The talkback control inputs activate the talkback relay K1,the mute relay K2 and signal the microcontroller to output the current talkback address. When one or more of edge connector pins 8, 10, 12 or 14 is grounded, pin 4 of U7B goes high turning on both Q1 and Q2 which activate K1 and K2.

The remote address control inputs activate the talkback relay K1, the mute relay K2, signal the microcontroller to read the corresponding preset address switch, change the display to reflect the new talkback address and output the new talkback address. When remote address 1 control input (edge connector pin 4) is grounded the talkback address is set to that of the preset address on SW1.

The mic on control inputs only activate the mute relay K2. When one or more of edge connector pins 7, 9, 11 or 13 are grounded, pin 8 of U8D goes high which turns on Q2 and activates K2.

The M718 can have up to 16 address selector keys. The keys should be wired in an eight row by two column matrix as shown in schematic C 718-02-001. The diodes are to prevent damage to the outputs of U3 in the event that two keys in the same column are pressed with one key being on a row that has a high output and the other on a row that has a low output. The address selector keys only select the current talkback address. The talkback address is not output until one of the talkback control inputs or remote address control inputs is grounded.

The busy tally input activates the open collector busy tally output. When edge connector pin 3 is grounded, pin 12 of U7F goes high and Q3 turns on.

Outputs

The talkback relay K1 has two form C contacts. The mute relay K2 has four form C contacts. Both are capable of switching 30 volts dc at 100 mA.

The busy tally output on edge connector pin 33 is an open collector output capable of sinking 30 mA.

The talkback address output is a four bit binary code with the LSB on edge connector pin 54 and the MSB on edge connector pin 48.

Talkback Address Keys

The address selector keys and their tally LEDs are multiplexed to reduce the number of inputs and outputs. To scan the keyboard, the 1 of 8 decoder U3 drives one of the rows (edge connector pins 39, 41 ... 53) low while the rest remain high. Edge connector pins 46 and 44 can only be pulled low by the switches connected to the row that is driven low by U3. Note that the display is turned off while scanning the keyboard by turning off Q4 and Q5. Thus for the microcontroller U1 to scan the keyboard it must set the upper 2 bits of its port B (B6 & B7) as outputs and keep them both high so that Q4 and Q5 remain off. Bits B4 and B5 are set as inputs so that U1 can read the status of edge connector pins 46 and 44. Bits B0, B1 B2 and B3 are set as outputs with B3 held low to enable decoder U3 and buffer U6A. B0, B1 and B2 select which row is driven low by U3.

See table 1 for the values that the microcontroller U1 writes to port B in order for U3 to drive a specific row of the keyboard low.

The display works in a manner similar to the keyboard in that U3 drives a single row low but in order to turn on an LED either Q4 or Q5 must be turned on. See table 2 for the values that the microcontroller U1 writes to port B in order to turn on a specific LED. Note that when B7 of U1 is low Q4 in turned on which enables the first column of the display. When B6 of U1 is low Q5 is turned on enabling the second column of the display.

Reading Inputs and Remote Addresses

In addition to scanning the rows of the keyboard, the microcontroller U1 reads the state of the talkback and remote address control inputs. U1 sets the upper four bits of its port B (B4, B5, B6 & B7) as inputs and the lower four bits of port B (B0, B1, B2 & B3) as outputs. U1 then writes a value to port B (see table 3) selecting the remote inputs by enabling U4A. The state of the talkback and remote address control inputs is then read on the upper bits of port B. If any of the talkback inputs are grounded then the current talkback address is output on port A of U1. (Buffer U6B protects the port A pins of U1 and provides more drive capability for the talkback address outputs.) If the remote address 1 control input (edge connector pin 4) is grounded then the microcontroller U1 writes a value (see table 3) to its port B to selec