This text is a condensed translation of the problems found with the EVM7000-3 board.

The goal of the corrections is to make the replica more compatible with the original EVM700 and the monitor revision 1.4. The first step is to get the replica working with monitor rev 1.4, which means the monitor runs and responds
to commands via the console. Any hardware for I/O like the programmer or Port emulation is not soldered to the board.

These fixes were needed:
- Disconnect the CPU A15 to SRAM, and connect IC40-31 A15 to ground. 
- The serial port labels P4 and P5 are incorrect. P5 is P1, the console on the original. P4 is P2, the auxilary port.
  The board text has RxD and DTR exchanged for both P1 and P2. The DTR ans DSR pins are used as handhake lines. This is mainly of interest for file downloads to the EVM. Note that the monitor uses a single software UART for both serial ports
- Some of the signal lines from the GALs are not very informative. The GAL definition files have other names.
- The crystal oscillator should be 4.1952 MHz. 4 MHz is within tolerance for the serial port baud rate.
- The barrel-jack connector has an incorrect orientation, and the holes are too small. (Should be elongated).
- IC26 pin 1 is incorrectly grounded. It should be connected to IC24-16 (PW3).
- IC19 is marked as 27256 for 32kx8 chips, but the monitor supports only up to 16kx8 devices. Larger devices wil require an modified monitor. The list from the manual: TMS2732, TMS2764, TMS27128, TMS27C64, TMS27C128, TMS7742, SEEQ72710, SEEQ72720. Note the manual version is newer than the monitor version.
- The GAL definition file GAL_IO_15.pld names pins starting with Emu... This should be Prog... as these control the latches for the Programmer.
  
Improvement points:
- A jumper to allow using a 28256 EEPROM instead of a 27256. A14 uses another pin.
- The (TTL-level) serial port RX and DSR lines need a pull-down for a reliable serial connection.
- The serial line signals RxD and DSR are not properly terminated, I added 100k resistors to ground for the inputs. 
  On the original EVM7000, the 1488/1489 drivers for the RS-232C interface regulated the proper levels, but with a
  direct TTL serial interface, this is somewhat different. Using a USB-serial interface with extra handshake lines
  works best and is also future proof; it allows hardware flow control for file transfer. For Windows either 
  RTS/CTS or DSR/DTR can be used, for Linux (and MacOS?) only RTS/CTS can be used. Make sure your terminal program 
  is configured for the correct hardware handshake. 
- IC8 74HC243 does not make much sense as replacement Port B input as for all TMS7xxx and 7xCxx port B is output 
  only. And with the outputs of IC7 74HC175 always enabled nothing new can be read anyhow. But it is part of the 
  original schematic...


Startup

The EVM7000 never runs directly at powerup. A reset and a received character are always needed (this is even in the manual). It helps to wait a few seconds between power up and reset, and reset and sending a <CR> (Enter).
When the monitor has a BAUD rate detected and listens to P1, the EXTRA0 LED is lit, the other threeLEDs are off.

If no prompt follows after sending a CR (Enter key, 0x0D), IC10-1 can be checked. This is the read pulse for the serial ports. It might help to pull the DTR line of P5/P1 low. It is Ok to connect it to the PC/terminal CTS. For complete hardware handshake (file download!), also connect the DSR to PC/terminal RTS.

If IC2-1 is high, the EVM listens to P1. If it is low P2 is active.


fjkraan@electrickery.nl, 2026-10-26