RCA CDP18S711 manual Using the Expansion Interface, Some Expansion Ideas

24 I RCA COSMAC VIP Instruction Manual

A single photocell input could be provided via the buffered EF4 line. You can attach the photocell directly between the L and Z pads. Experimentally adjust the pull-up resistor on pad L for best operation. No photocell amplifier should be required to drive the COS/MOS input. An, externally supplied positive pulse- on. Pins 2_and 14 of U25,can be used as an input,byte- strobe when yow want to latch an input byte into U25. A 68 can be used to store

this input byte in RAM.

also latch high-order address bits to select external devices if desired. When using external circuits to generate DMA requests, interrupt requests, or input flag signals, isolate these signals with 1N914 diodes as shown for EF3 and EF4 in the optional parallel input/ output port logic. Refer to the User Manual for the CDPI802 COSMAC Microprocessor, MPM-201A, for specific examples of input/output attachment techniques.

Using the Expansion

Interface

The 44-pin card socket for the expansion interface pads along the back left edge of the PC board permits extensive expansion. If you expand beyond the capabilities of the power converter provided with the VIP, you will, of course, have to provide your own power supply. Output signals should only drive COS/MOS loads, and must be externally buffered with a. CD4050 or CD4049 IC to drive TTL loads. Keep any wires connected to the expansion pad signals as short as possible. Excessive stray capacitance on these signal lines can interfere with proper operation of the computer. Input signals should also be buffered with COS/MOS circuits. Refer to the. machine language programming section (Section IV) and the logic diagrams (Appendix E) to avoid conflicts with normal COSMAC VIP use of these signals. The external option terminal connections are given in Table III.

You can latch up the required high order address bits with the trailing edge of TPA when adding external memory. You must provide a Positive level on pad 19 to disable internal RAM when external RAM is addressed. The operating system will always use the highest page of internal (on-card) RAM, even when you add external RAM.

If you wish to substitute an external ROM or battery-powered COS/MOS RAM for U10, you can use the signal on pad X to select it. Remove U10 when substituting an external ROM. If you do use an external ROM for your own operating system you may no longer be able to use the CHIP-8 interpreter because it requires some of the operating system subroutines.

The expansion interface pads provide access to all CDP1802 signals so that you can add any desired external circuits.

Only 5 out of the possible 14 CDP1802 input/output instructions are used internally, so that you can externally decode the N0, N1, and N2 lines and use them with MRD to obtain the use of the remaining 9 input/output instruction codes. You can

Some Expansion Ideas

The August and September 1976 issues of Popular Electronics contain descriptions of a COSMAC ELF microcomputer using the CDP1802. These articles illustrate some input/output attachment techniques.

The following lists some things that with some exercise of your ingenuity could be added to your system at relatively low cost:

1.Manually operated photoelectric paper-tape strip reader. Only requires a tape guide and 8 photocells.

2.Scanning circuit for multiple input lines from

sensing devices using CD4515 IC.

3.Full alphanumeric keyboard.

4.Low-cost printer.

5.Multi-digit numeric display.

6.Calculator chip.

7.Individual photocells or switches.

8.Output relays to control solenoids, bells, whistles, sirens, lights, or motors.

9.Sound-generating circuits that can be controlled by program.

10.Analog-to-digital input circuits.

11.Read-Only Memory for fixed program.

12.Digital-to-analog output circuits.

13.Alpha wave monitor input to control pictures on TV or output devices.

14.Temperature- or pressure-sensing devices.

15.Computer terminal.

16.A second hex keyboard for multi-player video games.