Introduction

When the EPA Micro-68 and the Motorola MC6800 Evaluation Kits were first introduced, it was common to have a teletype machine around. Because they were pretty noisey, they were usually kept in labs or hidden away in closets. The teletype served two purposes. First was to send and receive characters from/to the micro-computer. The other purpose was program storage. The teletype machines usually had a paper tape reader/punch. This meant that you could load a program into the micro-computer, and then save it by punching a paper tape. Then you could re-load the program any time you wish.

The paper tape library didn't have to get too big before it was hard to manage. Paper tape was expensive and difficult to store. If you had several revisions of the same program, you started to have a pretty messy library. Plus, users wanted to do their computer work in their offices, rather than dragging all your information back and forth to the lab or closet. RS-232 terminals were also available aroung that time, too. They were nice and quiet, but didn't have any utility to save your work.

Initially there were lots of different storage methods defined that used audio tones. But the problem was, none of them were compatible. So in late 1975 users and manufacturers got together and settled on a system that everyone could use. All you needed was a small interface circuit and a portable cassette recorder. This allowed the utilities, initially meant for operation with a teletype, to work with a cassette recorder.

While the TCC3 was intended to work with a cassette recorder, the cassette recorder is no longer needed. Now it is very easy to create a audio file on your PC. While the storage speed using the TCC3 will be slow (300 Baud max.), you no longer need cassettes. Managing files is much easier than paper tape or cassette.

What follows is directly from the user manual for the EPA Model TCC3 TTY/CRT/Cassette Adapter - K7MEM

EPA TTY/CRT/Cassette Adapter Model TCC3

1.0 General

The EPA Model TCC3 TTY/CRT/Cassette Adapter provides an interface for th Micro-68 computer to a standard 20ma current loop teletype, RS-232 CRT display and low-cost audio cassette simultaneously. The TCC3 adapter is permantntly mounted on the Micro-68 main circuit board just above the keyboard. TCC3 connection to peripheral equipment is made via the 36 pin edge connector on the side of the Micro-68. Audio cassette data is recorded in Byte Standard (Kansas City) format, up to 300 Baud. For convenience, 1200 Hz represents a zero and 2400 Hz represents a one on tape for all transfer rates up to 300 Baud.

2.0 Circuit Description

1. 14536 Baud rate generator. The MC14536 (U7) provides an adjustable pulse (9.1 ms for 110 Baud, 3.3ms for 300 Baud) to the 6800 MIK-BUG program for establishing proper serial data rates.
2. 555 Clock. The 555 timer (U3) provides a signal for the voltage inverter (CR1, CR2, C5, C10) and the SN74112 (U2) byte standard data encoder. the 555 is set or 4800 HZ with R2. One section of the LM3900 (U6) and the SN75154 (U8) provide signal conditioning into and out of the 4046. The center frequency of the 4046 is set to 1800 Hz with R3.
3. 75154 RS232 Receiver. The SN75154 (U8) interfaces RS-232 and teletype levels to TTL.
4. 4046 Phase Locked Loop. The CA4046 PLL (U1) converts the 1200 Hz and 2400 Hz signals to D.C. levels. Three sections of the LM3900 provide signal conditioning into and out of the 4046. The center frequency of the 4046 is set to 1800 Hz with R3.

3.0 Installation

If you have purchased the TCC3 adapter separate from your Micro-68 computer, proceed as follows:

1. Figure 1
   1)	+5V	6)	CB2	11)	PA7
   2)	PB0	7)	PA0	12)	CRT OUT
   3)	PB2	8)	TTY IN	13)	TTY OUT
   4)	PB7	9)	-5V	14)	READER
   5)	Ground	10)	CRT IN		CONTROL (RC)

   Solder a wire from the terminal marked "TAPE OUT" on the TCC3 board to pin "T" o the 36 pinedge connector.
2. Solder a wire from the terminal marked "TAPE IN" on the TCC3 board to pin "U" o the 36 pinedge connector.
3. Solder a wire from the terminal marked "+10V" on the TCC3 board to the positive terminal of the large 10,000 uF filter capacitor on the main Micro-68 board.
4. Solder 14 two inch lengths of heavy, bare wire to the 14 pads on the Micro-68 board corresponding to the following TCC3 board designations (see Figure 1):

5. Figure 2
   1)	CASSETTE IN--U	6)	CRT/MODEM out--M (L*)
   2)	CASSETTE OUT--T	7)	TTY READER CONTROL RELAY--C
   3)	TTY IN--F (E*)	8)	GROUND--H&J (F&H)
   4)	TTY OUT--E (D*)	9)	+5 Volts--K&L
   5)	CRT/MODEM IN--N (M*)	10)	-5 Volts TTY COMMON--D (C*)
   		11)	STOP BIT CONTROL--16
   * revision "C" and			(GROUND FOR 300 BAUD,
   earlier Micro-68			LEAVE OPEN FOR 110 BAUD)

   Carefully thread the heavy wires through the 14 corresponding pad holes in the TCC3 adapter board. Position the adapter board about one inch above and parallel to the Micro-68 board. Solder all 14 connections. Leave 1/4" excess wire above te adapter board on these connections for test points.
6. Connect the appropriate peripherals to the 36 pin edge connector as desired (See Figure 2)
7. Ensure that a jumper wire is soldered between pins 2 and B of the 36 pin edge connector.

4.0 Adjustment

The TCC3 adapter board is factory adjusted for 110 Baud. To check or readjust, proceed as follows:

1. Tape out level (R1). Set the tape output level for approximately 0.5 volts Peak-to-Peak for microphone input to a cassette recorder, 3 volts Peak-to-Peak for line inputs on higher quality recorders. The general rule of thumb is to set R1 so that the VU meter (for recorders so equipped) reads zero DB with the record level control set at its mid-point. For recorders without meters, keep the recorder level control relatively low. Proper recording level is important. Incorrect level can be determined by listening to the tape. Listen for extreme distortion or high noise.
2. Encoder frequency (R2). Set R2 so that the signal at U3 pin3 has a period of 208 µs (4800 Hz).
3. Phase locked loop center frequency (R3). Set R3 so that the signal at U1 pins 3 and 4 has a period of 555 µs (1800 Hz).
4. Baud rate. Set R35 so that the signal on U7 pin 13 has a low portion of 9.1 ms for 110 Baud and 3.3 ms for 300 Baud.

5.0 Operation

1. Teletype or CRT operation. Set PA7, PB7 and TTY switches to the "ON" (+) position and the reader control switch to the "OFF" position. This will result in all output being echoed on the CRT or TTY. Setting the reader control switch to the "ON" position will suppress printing during load with the MIKBUG if your TTY or CRT has this option installed or if the reader control line is used for this purpose. To record on a cassette, follow these steps:
   1. Set dump addresses in A002-A005 (if using the MIKBUG)
   2. Turn the cassette recorder to "record and set record level.
   3. After sufficient cassette leader, depress "P" on the CRT or TTY to initiate dump or insert a paper tape in the TTY reader and type "L" on the TTY to initiate load. After load is complete, type "S9" to terminate the load.
   Note: If your cassette player feeds back input to output during recording, it will be necessary break this loop by disconnecting the output from the recorder during recording. To load from a cassette, follow these steps:
   1. Type "L" to set the MIKBUG in the load mode.
   2. Set the tape playback level for a relatively high output and begin playback. No echo will occur if the reader control (RC) switch is in "ON" position.
   3. Type "S9" to terminate the load mode in the MIKBUG. An asterisk indicates a good load. A check sum error prior to completion of the load will cause garbage to be printed until the cassette player is stopped.
   Refer to Motorola Engineering Note 100 Excerpts on the last pages.
2. Micro-68 operation without CRT or Teletype. To enable the Micro-68 hexidecimal keyboard, set the PA7 and PB7 switches to the "OFF" position. Setting the PA7 and PB7 switches to the "ON" (+) position will connect the computer output to the TCC3 adapter, but will disable the "7" and "F" keys on the keyboard.

6.0 Trouble Shooting

1. Check for +5 (4.5 - 5.2) volts and +10 volts (7-10) on the TCC3 board. If these are not present, check for proper wiring.
2. Check for -5 volts (-5 to -3) on the TCC3 board. IF this oltage is absent, check for proper encoder oscillator operation.
3. Check for proper encoder oscillation operations. See paragraph 4B.
4. Check for proper phase locked loop oscillatio operations. See paragraph 4C.
5. Check for proper Baud rate generator operation. See paragraph 4F.
6. If these steps fail to determine the cause of trouble, it is recommeded that the TCC3 adapter be returned to the factory with the computer for repair at nominal cost.

After the section on Trouble Shooting (Section 6), Section 4 from the Motorola Engineering Note 100 is inserted. Rather than insert the same material here, please reference the on-line HTML Engineering Note 100 (HTML) , Section 4.0 or view the PDF at Motorola Engineering Note 100 (PDF), Section 4.0.

To operate the Micro-68 in a Stand Alone Mode

1. Make sure that the following chips are installed as indicated:
   7430 in socket U15
   Mon-1 Bug 1L in socket U10
   Mon-1 Bug 2L in socket U13
   Mon-1 Bug 1M in socket U17
   Mon-1 Bug 2M in socket U20
   PIA's (6820) in sockets U25 and U26
   MPU (6800) in socket U11
   Either a RAM (6810) in socket U16 or auxiliary memory at address 0000
   (switch on memory board set at 1010100) but not both
   Remove the MIKBUG (6830L7) from socket U19
2. If a TTY adapter is installed (the piggy back board above the keyboard), set the TTY adapter switch as follows:
   1 (PB7) - Off
   2 (PA7) - Off
   3 (RS232) - either Off or On
   4 (TTY) - either Off or On
3. Depressing the "0" and "4" keys simultaneously should yield the message "EPA UP" on the LED's (if you plugged in the power cord transformer to a wall outlet first).
4. Please refer to the Micro-68 User's manual and the teletype user's manual for additional information.

EPA Cassette Adapter - Timer and Power/Ground

The MC14536 device is used as the interface timer. This timer's interval is set by adjusting the 50K Ohm resistor (R35) and monitoring the output signal on pin 13 of the MC14536 device. The zero level of the timing pulse should be 9.1 ms for 10 characters per second (CPS) operation and 3.3 ms for 30 CPS operation. Also, pin 16 (PB6) of the MC6820 PIA should be connected to +5V for 10 CPS operation and grounded for 30 CPS operation.

At the bottom of the page is the Power/Ground wiring. Note that there are eight 0.1 µF bypass capacitors. One for each IC. These capacitors should be physically mounted as clost to the Power/Ground pins of it's associated IC.

EPA Cassette Adapter - TTY/CRT Input/Output

The 75154 RS232 Receiver (U8) converts the TTY and CRT input levels (Rs-232) to TTL for . The SN75154 (U8) interfaces RS-232 and teletype levels to TTL.

The 555 timer (U3) provides a signal for the voltage inverter (CR1, CR2, C5, C10) and the SN74112 (U2) byte standard data encoder. the 555 is set or 4800 HZ with R2. One section of the LM3900 (U6) and the SN75154 (U8) provide signal conditioning into and out of the 4046. The center frequency of the 4046 is set to 1800 Hz with R3.

EPA Cassette Adapter - Phase Locked Loop and Filters

This section consists of a Input Amplifier (U6-D) section, a Phase Locked Loop (U1) section, an Schmidt Trigger (U6-D) section, and a Comparator (U6-B) section.

The input capacitor (C14) provides DC isolation between the Tape Input (Tape In) and the Input Amplifier stage (U6D). The Input Amplifier (LM3900 - U6D) is configured as a amplifier with a gain of R18/R13 = 1M/100K = 10. With these values, R20 should be 2×R18 = 2×1M = 2M. The output DC voltage (U6D-10) will the be +V/2 = +5V/2 = 2.5V.

The output of the Input Amplifier (LM3900 - U6D-10) feeds directly into Phase Comparator Ⅰ of the CA4046 PLL (U1). The output of Phase Comparator Ⅰ is then passed through a Low-Pass Filter (R7, C3) and into the Voltage Controlled Oscillator (VCO) Voltage Input (U1-9). The components R3, R4, and C2 set the center frequency of the VCO to 1,800 Hz. The output of the VCO (U1-4) is then fed back to the other Phase Comparator Ⅰ input.

With this configuration, any change in the input frequency (U1-14) causes the VCO to adjust its output frequency (U1-4), which then matches the change in input frequency. The Tape In signal should be swinging from 1200 Hz and 2400 Hz. This will, in turn, cause the VCO to swing between 1200 Hz and 2400 Hz. The Demodulator (DEM) output (U1-10) is a DC signal that will swing between 0 V and +5V, as the input frequency changes

The CA4046 PLL (U1) converts the 1200 Hz and 2400 Hz signals to D.C. levels. Three sections of the LM3900 provide signal conditioning into and out of the 4046. The center frequency of the 4046 is set to 1800 Hz with R3.