RS232c / PIO interface card


Blow-up of RS232/PIO board

* This card was modified so as to answer at CRU >1500. This lets me have two RS232 cards in my PE-box. See below for details.


PE-box connector

  • Connections #2 to 60 on this side, connections #1 to 59 on back side
  • 244: Three 8-bit buffers, 74LS244 for the address bus and control lines.
  • 245: A 8-bit transceiver, 74LS245 for the data bus.
  • 125: A quad 74LS125 buffers, for 3-state output signals (e.g. RDBENA* , EXTINT*).
  • 7805: Voltage regulator, TL780-05C, +5 volts. Three 47 uF bypass caps (C12, C40, C41).
  • 7812: Voltage regulator, 78M12C, +12 volts. Two 47 uF bypass caps (C13, C38).
  • -12: Negative voltage regulator, 79L12A. Two 47 uF bypass caps (C18, C100).



    • CRU logic

  • 259:An 8-bit addressable latch, 74LS259 for CRU output.
  • 251: An 8-to-1 multiplexer, 74LS251 for CRU input operations (unbuffered CRUIN line to the PE-box).
  • T: A transistor NP2N3904 to control the LED.
  • LED: A yellow light-emitting diode controlled by CRU bit 7 (not bit 0 as usual!).


    • Memory

  • ROM: A 4K ROM, labelled CD3235A-NL DBT8240

    • Selection logic

  • Custom: Custom control chip, labelled I500280-3 RS00MMI8245. Selects the ROM (pin #13), the PIO port (pin # 14 for PIO in, pin #15 for PIO out), CRU chips (pin #16), and TMS9902 chips (pin #17 for RS232/1, pin #18 for RS232/2).
  • 00: A quad NAND gates, 74LS00. One gate not used.
  • 02: A quad NOR gates, 74LS02.
  • 21: A dual 4-input AND gates, 74LS21.
  • 32: A quad OR gates, 74LS32.


    • PIO port

  • 373: Two 8-bit latches, 73LS373. One for input, one for output.
  • PIO: Parallel port connector, 16 pin. Odd pins on top, even pins at bottom.


    • RS232c port

  • 9902: Two UART chips, TMS9902. Each handles one serial port.
  • 084: A TL084ACN quad op amp, -12 to +12 volts. Handles TX and DCD output lines.
  • 082: A TL082CP dual op amp, -12 to +12 volts. Handles CTS output lines.
  • 189: A 75189 quad inverters. Handles RD and DTR input lines.
  • RS232: DB-25 connector combining the two RS232C ports in a single plug. Pin 1 to 12 on top, 14 to 25 at bottom.


    • CRU modification

    This card is designed so that two cards can be installed in the PE-box, one with CRU >1300, the other with CRU >1500. The DSRs are written so as to handle this situation and provide two parallel ports and four serial ports.

    Unfortunately, instead of using a simple switch, TI designed the card so that a resistor must be moved to change the CRU. This resistor is identified as R5 (see the green star * in the picture above) and must be moved in the position labelled PTH1 (just below it).

    Here is a blow-up of the modification:

    Blow-up of CRU modification

    The resistor was initially in position R5, just below the 74LS02. I desoldered it and transplanted it to position PTH1. This connects pin19 of the custom control chip (on the right of the picture) to +5 volts, instead of ground.


    Pin assignment modifications

    TI also made provisions for us to change the pin assignments for the DTR lines. Normally DTR-1 is pin 20 (jumper W5) and DTR-2 is pin 19 (jumper W6). However, It is possible to assign DTR-1 to pin 11 instead, by cutting W5 and replacing it with a jumper at W7. Similarly, DTR-2 can be assigned to pin 18 by cutting W6 and wiring W8.

    The location of these jumpers is flagged by two white stards ** on the main picture. Note that jumper W7 is a bit further up.

    The above card was not modified in such a way, as I'm not sure what this modification is meant for.



    For further details, see my RS232c page.


    Revision 1. 8/26/01. OK to release.


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