Model 1 Overview

The Model 1, in it's Level II BASIC configuration, consists of a Z-80 processor running at 1.77 MHz, 4 to 48K of RAM (32K of that being housed in the optional Expansion Interface), and 12K of ROM. The 16 line by 64 character video is memory mapped from 3C00H to 3FFFH. The keyboard is memory mapped from 3800H to 38FFH, using the layout in the table below. When the expansion interface is connected, there are several other items available. A printer port is memory mapped at 37E8H, with data sent to the printer on write and status available on read. The floppy disk controller takes up five addresses in the 37E0H to 37EFH, as detailed below. In addition, there is an interrupt that can be used as a time of day counter. Finally, the expansion interface allowed installation of an RS-232 communications board, though the connection was "flakey" at best.

The back of the Model 1 had four connectors. Three five pin DIN jacks that were used for Power, Video and Cassette, and a 40 pin card-edge connector that was used to connect peripherals (such as the Expansion Interface) to the computer. Pinouts of all these are listed below.

Model 1 video connector

The TRS-80 Model 1 monitor was actually a black and white RCA television set with most of the guts taken out. Because the chassis was connected to the power mains, Radio Shack added an optical isolator to help prevent shocks. The output of the TRS-80 was 5 volts to power the optoisolator, and a standard 2 volt composite video signal. If you want to have a bigger monitor, you can wire up a DIN connector with pins 4 and 5 and connect that to the video input of your TV (or VCR or whatever).

The pin connections on the Video DIN plug are:

         2               1 - 5V dc (30ma max)
     5   o   4           4 - Composite video
      o     o            5 - Computer ground
   3 o       o 1

Model 1 Power connector

The pin connections on the Power DIN plug are:

         2               1 - 14 VAC @ 1A transformer
     5   o   4           2 - 19.8 VDC @ 350 ma
      o     o            3 - 14 VAC @ 1A transformer
   3 o       o 1         4 - Ground
                         5 - unused

The Model 1 power supply consists of two center trapped transformers. There is a 14 volt AC transformer that goes between pins 1 and 3 of the connector. This is rated at 1 amp. Pin 4 gets the center tap of that transformer. The other transformer is half-wave rectified and Pin 2 gets approximately 19 volts DC at 350ma for the +12VDC regulator.

A simplified ASCII schematic of the Model 1 wall wart transformer is shown below:

   ________  _____________________________ Pin 1
  |        )(              
 115       )(_____________________________ Pin 4
 vac       )(
  |________)(_____________________________ Pin 3
            
                 1N4000
            (----->|-----------------*---- Pin 2   
            (                        |
            (-----------> to pin 4   |
            (                        |
            (----->|-----------------
                 1N4000

Model 1 Cassette Connector

         2               1 - Motor relay
     5   o   4           2 - Ground
      o     o            3 - Motor relay
   3 o       o 1         4 - Cassette In
                         5 - Cassette Out

The Motor relay contacts connect to the remote control jack on the cassette to turn the motor on and off (duh!). Cassette In goes to the earphone jack of the recorder. Cassette Out should go to the Aux jack of the recorder, if it has one. If not, to the mic connector. The level may be a little high if you have to go through a microphone input, though.

40 Pin Expansion Connector

The Model 1 has a 40 pin bus connector on the left rear of the case, which allows connecting various peripherals. It is mainly used for the Expansion Interface, a box containing an additional 16 or 32K of RAM, a floppy disk controller, a printer port, and possibly an RS-232 port.

The expansion bus pinout is as follows:

Expansion Bus Card Edge Connector as viewed from the rear of the computer.

  1  3  5  7  9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39
 -#--#--#--#--#--#--#--#--#--#--#--#--#--#--#--#--#--#--#--#-
  2  4  6  8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40

Explanation of Expansion Bus signals

Address Output: There are 16 address lines, labeled A0 through A15. A0 is the least significant bit, while A15 is the most significant bit. These lines are the address bus from the Z80 microprocessor. Each line is capable of driving ONE standard TTL load.

Bi-directional Data Bus These eight lines, D0 through D7, are what the CPU uses to move data from one part of the computer to another. D0 is the least significant bit, while D7 is the most significant.

Row Address Strobe output The RAS* line goes low when the CPU is outputting the row portion of an address. Used for accessing dynamic RAM.

Column Address Strobe output The CAS* line goes low when the CPU is outputting the column portion of an address. Used for accessing dynamic RAM.

Multiplexer Control output The MUX output is used to select the proper address line in conjunction with RAS* and CAS* for accessing the RAM.

System Reset output The SYSRES* output goes low only when the RESET button is pressed, or when the computer is first powered up. This can be used to reset an external device at the same time the TRS-80 is reset.

Test input When the TEST* line is taken low, the data, address and control group buffers will tri-state. That is, they will be disconnected from the rest of the world. This is normally only used during factory testing or during troubleshooting.

Processor Wait When taken low, WAIT* will pause the CPU from firther processing until WAIT* goes back high. In some cases, an external device will need additional time to send data to the processor. The WAIT* input allows the external device to take the time it needs.

Memory Write strobe When *WR goes low, the CPU is writing the data present on the data bus to the memory location specified by the address bus.

Memory Read strobe When *RD goes low, the CPU is reading data present on the data bus from the memory location specified by the address bus.

Peripheral Write strobe OUT* operates like WR*, except that it is for port output instead of memory write. When OUT* goes low, the CPU is trying to send the 8 bit data on the data bus to the I/O port specified by the 8 low order bits of the address bus (A0 through A7). The Model 1 can address up to 256 output ports.

Peripheral Read strobe IN* operates much like RD*, except it is for input ports instead of RAM memory. When IN* goes low, the CPU is looking for data at the port address specified by the low 8 bits (A0-A7) of the address bus. The Model 1 can address up to 256 input ports.

Interrupt input INT*, when taken low, will force the CPU to a predetermined address in the computer's ROM. Although the Z80 CPU allows several interrupt modes, there is only one available on the TRS-80, the jump to 0038H.

Interrupt Acknowledge INTAK* goes low whenever the the CPU enters an interrupt mode.

Signal Ground This is the reference point for all voltages and logic levels in the Model 1.

+5 volt output This line is at +5V only on Level 1 Model 1s. On Level II machines, this pin has been modified to be GROUND.

Model 1 Memory Map

The Model 1's Memory Map included the BASIC ROM, Memory mapped I/O regions for the keyboard, printer and floppy disks, video RAM, and system RAM. The table below shows the address ranges for each of these.

Model 1 Keyboard Memory Map

The Model 1 keyboard was memory mapped from addresses 3801H through 3880H. The table below details each address and the keys that were mapped to them.

Notes on the table:

• Only unshifted values are in the table. For the characters that correspond to the shifted keys, consult your Model 1 or 3 keyboard.
• Blank entries are not used.
• The BREAK key can not be used in BASIC programs.
• It is entirely possible to check for multiple keys at once, at the same address. "H" and "I" pressed at the same time would give the value of 3 at address 3802H.
• It is also possible to combine more than one address. Add the low bytes of the desired row addresses together, and add that value to 3800H to find the address to peek. For example, to check if either "H" or "X" is pressed, check address 380AH for a value of 1.
• The box marked ***** shows the address and bit used on the Model 3 to decode the right shift key separately from the left shift key.
• Electric Pencil used the "Control" key at 3880H. It wasn't a standard Model 1 or 3 key, though.

Model 1 I/O Ports

The Model 1 used only two I/O ports. The cassette port was at 0FFH and the serial port mapped from 0E8H to 0EBH.

Thanks to Robert Brooks, we now have a nice table showing the Model 1 Port assignments. Thanks much, Robert.

Model 1 DOS Internals

These are rather large to put here. Please use the links below to see this information.

Model 1 and 3 DOS Call addresses

Model 1 LDOS Functions

Miscellaneous

Miscellaneous info goes here... when I get time.