This "challenge" covers the provision of an 8 bit output port driven by a
single optically coupled digital output line. Constraints such as lowest
cost and PCB area add interest to the requirement.

I have to produce 4 to 8 digital output lines from an RS232 level "RTS"
output. (At least 4 outputs required - more would be useful). The so called
RTS line is in fact a general purpose digital output line. The client
requires the line (and an accompanying RS232 serial data line) to be
optoisolated from the target - mainly to prevent the equipment acting as an
earth path for the heavy currents drawn by the target equipment.

The final product (which has a other functions not relevant to this
"challenge") mounts inside a VHF R/T (radio telephone) immediately above the
BNC RF aerial connector. This doesn't seem to pose excessive problems in
prior versions of the application. Present final volume is moderate
(hundreds) BUT there is a prospect for a very large volume follow up
(100k?). Data transfer rate is not critical. Probably a data word sent in 10
to 100 ms when required would suffice. Faster is acceptable.
Operation from 5 volts. CMOS (wimpy) output drive levels OK.

Key design constraints are

    - lowest cost
    - smallest PCB area (within reason).
    - Doesn't affect R/T operation
    - works

The obvious solution is a UART. All that I have found fail the cost
requirement substantially when compared with alternatives.

The next most obvious is a single chip microprocessor. These too seem too
dear for the task. Even a 12C508 or 16C505 are far dearer than hardware
alternatives ($US1+)(Z8PExxx down to about $US0.90). The need for
programming adds slightly to the cost. Other brands seem hard pressed to get
under about $US1. Some of the more exotic Asian 4 bitters would probably
halve this. The anguish of pursuing this path MAY be justified for the 100K
solution, maybe.

Using the data line plus the accompanying RS232 line and implementing IIC
could be a solution. Circuitry to prevent the RS232 line burbling during
this time would add to complexity. (A matching return pair of optically
isolated data and control lines is present. It had not been expected that
these would be used in the output solution but this is a possibility. If so,
their existing functionality would have to remain. (RS232 RX and 1 digital
input).

The current "winner" is a CD4094 serial to parallel shift register plus 3
CD40106 inverters (half a pkg) plus a few R's and C's to derive clock, data
and latch signals from a single serial line. Positive going pulses are sent
from the controller. Pulses over a certain duration are seen as 1's, less
than this are 0's. (0 = nominal 1 ms high, 1 = nominal 3 ms high. Barn door
accuracy timing easily separates these. In practice 0's may be much shorter
if desired. As the data feed is interrupt driven these timings would be
used. Cessation of data for more than a certain period (say 10 ms) results
in latching of the data sent. ICs would be SOIC pkgs. Smaller not really
needed. The spare inverters will be used in other circuitry. Parts cost is
about $US0.40 (4094 + 40106/2 + glue).

This solution could be improved for the digital purists by using a local
clock and gated timing to replace the nasty analogue delays at the probable
expense of several extra ICs.

Any better ideas?



        Russell McMahon

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