RS232 for Sharp pocket computers

Programming has an interface that can be connected to a special cassette interface. The signals at that interface are very similar to those of an RS232 interface, but they are inverted and have different logic levels. It would, of course, be useful if the pocket computer could be linked to a larger computer via this interface, because the writing, changing and storing of the software could then be done much more conveniently. It only requires the small circuit down to make that possible. The single 5 V supply voltage is converted into ±10 V in 1C1. With these voltages, the buffers in IC1 can convert the logic signals of the pocket computer into RS232 levels. Inversion of the levels is effected by four inverters in IC1. The circuit draws a current of only 30 mA so that it can be supplied from the larger computer.

The connector to link the interface to me pocket computer presents a small inconvenience because its pitch (1.27) is rather unusual. Connectors with that pitch are normally too long for this application; the only solution is to cut one to size. The circuit has been tested with an XON/XOFF protocol, 2400 baud, even parity, 8 data bits and a stop bit. At higher speeds, small problems arose, but that need not always be the case, depending I on the software.
The interface of the Sharp pocket computer is set with:
OPEN” COM:2400, E, 8, 1, A, L, &H1A, X, N”: CLOSE.

RS232 for Sharp pocket computers Schematic diagram

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