80-Bus News

and A13 (also A14 for future expansion), control lines /OE, PGM and /PGM, and power supplies Vcc (+5V), Vpp (+21V and +25V) and GND (0V) are taken to a 16-pin dil socket. ZIF socket connections that vary depending on the EPROM being used are also taken to this socket. By inserting a header plug with the required connections the appropriate EPROM may be programmed. See fig 2 for header connections. Note that some EPROMS require a low to high programming pulse whereas others require a high to low pulse – hence the use of two signals PGM and /PGM respectively.

The +5V supply is provided by a 7805 5V regulator from an unregulated 30V supply. The two Vpp voltages are supplied from two separate circuits built around two 723 regulators. Although the 723 can deliver the 30ma Vpp current I included a bypass transistor to take most of the dissipation away from the 723. The function switch has 3 positions– OFF, READ (Vcc and Vpp at +5V) and Program (Vcc at +5V and Vpp at +21 and +25V). Two LEDs indicate when Vpp and/or Vcc is applied. The 5.1V zener stops the Vpp LED lighting when in READ mode (Vpp=+5v).

Control Program

By restricting the EPROM types that can be programmed, and routing the signals via the header socket, the control program has been made very straightforward and, apart from checking to see if a 2532 is being programmed, it does not really need to know which EPROM is in the socket since the routines and control signals are the same. However, for convenience of the user you are requested to enter the EPROM type so you will be told if you are trying to program a 4K block of memory into a 2716! It also uses the EPROM type information to work out the number of locations to check for erasure and to read in the EPROM contents to RAM.

On entering the program a menu gives the options:–

1. Verify EPROM fully erased
2. Read in EPROM to memory
3. Program EPROM (and verify)
4. Return to NAS-SYS

The basic structure of each operation is a simple loop which outputs an address, sets the control lines to perform the operation, either writes data from RAM to the EPROM or reads data from the EPROM into RAM, increments the EPROM address and RAM pointer, and continues until the operation is complete. In each case you will first be asked to enter the EPROM type. The previously entered type is displayed and if this does not need to be changed just press Newline.

1. Verify

This routine reads in every location from the EPROM and compares it with FFh which signifies that it is erased. If the location is not erased, a message is displayed together with the address of the offending location. If there are a lot of unerased locations you can press ESCAPE to abort the verification. Then you may try another EPROM or return to the menu.

2. Read in to RAM from EPROM

You are first asked for the start address of the RAM block where you want the contents of the EPROM to be dumped. A default address is shown so just press Newline if this does not need to be changed. The whole of the EPROM will