TRS-80 Model 4/4P I/O Interfacing

Serial Port

The serial port uses a TR1865, TR1602, or equivalent UART. A compatible UART, the HD-6402,
is also manufactured by Intersil; ... it might be intersting for reference:
• HD-6402 summary -- http://www.intersil.com/cda/deviceinfo/0,1477,HD-6402,0.html
• HD-6402 PDF datasheet -- http://www.intersil.com/data/fn/fn2956.pdf

  This type of UART is unique because it has separate baud generators for Tx and Rx
  (transmit and receive can have different baud rates). For example, you can send
  data at 19200 Baud, but receive data at 9600 baud. Each line is configurable for
  independent baud. The Model 4/4P are the same, and the maximum baud rate is 19200.
  

  TRS-80 Model 4 RS-232 Port Definitions

  Port E8h
  E8h Output: UART Master Reset, enables UART control register load
  
  • d1 when '1' enables control register load (see Port EAh description below).
  Input: Modem Status Register
  • d7 Clear-to-Send (CTS), Pin 5
  • d6 Data-Set-Ready (DSR), pin 6
  • d5 Carrier Detect (CD), pin 8
  • d4 Ring Indicator (RI), pin 22
  • d3..d1 Not used
  • d0 ?? UART Receiver Input, pin 20 (pin 20 is also DTR)
  Port E9h
  • E9h Output: Baud-rate Generator Register load enable bit

    data	Baud
    00h	50
    11h	75
    22h	100
    33h	134.5
    44h	150
    55h	300
    66h	600
    77h	1200
    88h	1800
    99h	2000
    AAh	2400
    BBh	3600
    CCh	4800
    DDh	7200
    EEh	9600
    FFh	19200

  • E9h Input: Reserved (Not Used)
  Port EAh: UART Control Register Load and Modem Control
  • EAh Output (Write), when E8h bit d1 is '1' (Control Register enabled):
    bits d7..d3 are UART control; d2..d0 are Modem line control
    • d7 Even Parity Enable ('1'=even, '0'=odd)
    • d6 Word Length Select 1
    • d5 Word Length Select 2
    • d4 Stop Bit Select ('1'=two stop bits, '0'=one stop bit)
    • d3 Parity Inhibit ('1'=disable; No parity, '0'=parity enabled)
    • d2 Break ('0'=disable transmit data; continuous RS232 'SPACE' condition)
    • d1 Data-Terminal-Ready (DTR), pin 20
    • d0 Request-to-Send (RTS), pin 4
  • EAh Output (Write), when E8h bit d1 is '0' (Control Register disabled): bits d5..d3 are secondary line control, and could even be used for steering or multiplexing logic; d2..d0 are Modem line control same as above
    • d7,d6 Not used
    • d5 Secondary Unassigned, pin 18
    • d4 Secondary Transmit Data, pin 14
    • d3 Secondary Request-to-Send, pin 19
    • d2 Break ('0'=disable transmit data; continuous RS232 'SPACE' condition)
    • d1 Data-Terminal-Ready (DTR), pin 20
    • d0 Request-to-Send (RTS), pin 4
  • EAh Input (Read): UART Status Register
    • d7 Data Received ('1'=condition true)
    • d6 Transmitter Holding Register empty ('1'=condition true)
    • d5 Overrun Error ('1'=condition true)
    • d4 Framing Error ('1'=condition true)
    • d3 Parity Error ('1'=condition true)
    • d2..d0 Not used
  Port EBh Output (Write): UART Transmit Holding Register
  • An output to this port load the UART transmitter Holding register with a word to be
    transmitted, as soon as the last word loaded in the holding register is transmitted.
    This register should never be loaded until the Transmitter Holding register empty bit
    (Port EAh) is true.
    
  Port EBh Input (Read): UART Receive Holding Register
  • An input from this port reads the last word received from the UART received
    data holding register. This register should not be read until the data received
    bit (port EAh) is true.
    

  Setting the Baud Rate:

  The TR1602 UART is clocked by a Dual Baud-rate Generator chip, (Western Digital BR1941L,
  or equivalent) which creates two separate baud clocks. The baud is written as one byte
  which contains two sets of 4-bit codes, using 4 bits to select Tx baud, and another 4 bits
  to select Rx Baud. Usually you set both Tx and Rx baud to the same. Tx baud is set by the
  upper 4 bits (d7..d4), Rx Baud is set by the lower 4 bits (d3..d0)
  

  Refer to Port E9h Output description above, and write the desired datum to port E9h.
  

  Setting the protocol:

  Usually you will use '8-N-1' (as 8 data bits, No parity, 1 stop bit)
  • write 02h (bit d1='1') to port E8h to enable Control Register write
  • write the desired configuration byte to port EAh
  • Note that d6:d5 are Word length Select d5=WLS2, d6=WLS1
    • d6='1',d5='1' for 8 bits
    • d6='0',d5='1' for 7 bits
    • d6='1',d5='0' for 6 bits
    • d6='0',d5='0' for 5 bits

  Parallel Port:

  The printer port can also be used for I/O control...
  • F8h Write (output) send data to the printer data lines with strobe pulsed low.
    
  • F8h Read (input) reads printer status as follows:
    • d7 BUSY ('1'=true)
    • d6 Out-of-Paper ('1'=true)
    • d5 Device Select ('1'=true)
    • d4 ERROR, printer fault ('1'=true)
    • d3..d0 Reserved

  Model 4/4P I/O Bus:

  The I/O Bus is a very flexible way to interface a Z80 system to the outside world, but
  requires a thorough understanding of the Model 4 I/O bus circuitry and the required
  external interface circuitry. For example, it requires proper buffering and a strong
  driver on input signal EXTIOSEL for reading data from the expansion bus. The entire
  I/O Bus also must be 'activated' before use. ... More about this later.
  

  
  

  
  

  
  EOF