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When a byte of data is received, the Strobe- line will pulse low as described previously. This is connected to the Write Enable- (pin 27) on the RAM IC's so the received data will be written to the current memory location.

The Strobe- signal is also connected to pin 23 of the UART. This will cause the transmission section of the device to go through it's motions (even though nothing is listening!). As the data is sent, pin 24 will pulse, incrementing the address counters IC5 and IC6. The section of the UART is used to merely apply a suitable delay before the address is incremented, to ensure the data is correctly stored.

When the next byte of data is received, it will be loaded into the next memory location, and so on. As long as the user presses the Reset button when prompted by the software, the data will arrive in the correct places. The software simply sends the data to the emulator one byte at a time, you could alternatively simply copy a binary image file to the COM port.

Two 256K x 1 Bit (32K x 8 Bit) RAM devices are used, to obtain the full 64K addressing range of a 27512 EPROM. The CS lines are used in conjunction with an inverter on the A15 line to select which device is used at a particular time.

Now assume that SW2 is in the lower (Run) position. IC7, IC8 and IC11 are now disabled, isolating the RAM from the upload circuitry. IC12 and IC13 are enabled, effectively connecting the RAM address bus to the 'EPROM' socket. SW3 and SW4 select the EPROM type, and set any unused RAM address lines low. When pins 22 and 24 of the 'EPROM' are both low, IC14 is enabled (via IC4) connecting the RAM data lines to the 'EPROM'.

The prototype has not yet been tested on a speed-critical microprocessor system. The interface electronics should add no more than 20ns to the access time of the RAM devices used. Therefore with 100ns RAM chips, the unit should be able to emulate a 120ns EPROM, and will definitely emulate a 150ns device. If speed is critical, fit the fastest RAM chips you can get - 35ns devices are available if you are prepared to pay for them!

Construction
The unit is assembled on a single sided PCB. A number of wire links are required, which should be fitted before any components, since some pass underneath ICs. I would suggest that the resistors are fitted next, followed by the ICs, then the capacitors, then the remaining parts. Fit a link wire between COM and 96 to set the Baud rate to 9600. Fit SIL header strip or Veropins for the off-board connections.

I would suggest that sockets are used for the RAM chips and the UART, in view of their cost. It would also be a good idea to use sockets for IC4, IC12, IC13 and IC14, since these interface to the outside world and could be damaged if there is a problem on the system being tested. The sockets allow for ease of changing if the worst happens.

If you do not plan to emulate 27512 EPROMs, you could save a few pounds by omitting IC10 (it can always be added later). The unit will then emulate up to 27256 devices, so move the stop on the Device switch to prevent the 27512 option being selected.

Fit a 28-way IC socket in SK2 position. A 300mm (or shorter) length of 28 way ribbon cable should be fitted with a 28 way DIL connector on each end. These can be readily pressed together in a vice or WorkMate, if three thicknesses of Veroboard are used to protect the connector pins. Take care when doing this, as it is not easy to get the connector apart again if something goes wrong. One end of this cable plugs onto SK2, make sure the edge of the cable with the different colour goes to pin 1.

When you receive these connectors from your supplier, please make sure the two parts are not pressed fully together before removing from the polythene packing. If they are, send them back. The two I purchased from Maplin were fully assembled and I managed to break one while disassembling them.

The interwiring is shown in figure *. This should be carried out at this stage, since it is necessary for testing. After testing the board can be fitted into the case.

The connections for both 9 and 25 way serial connectors, use whatever matches the socket on your computer. On the prototype a 9 way D connector (serial) and a 6 way DIN socket (DC input) were fitted to the case. The rotary switch connections are shown by giving the pin number or letter marked on the switch body.