Explorer 85


Although this system represents a DIY start to S100 computing it may not be the best value for money. CT reveals the inner secrets.

By David Sinfield

The Explorer 85 from Newtronics Research and Devel­opment Ltd, USA, (the people who brought you the ELF II, reviewed in CT October 1979) has been available in this country for some time. It has, to date, attracted little attention. It is based on the 8085A processor – very similar to the 8080 and software compatible with that popular device. The CPU board is 13.25x 10.75” and of a high standard of manufacture being glass epoxy, double sided, through hole plates, silk-screened and solder masked (phew!). Fully expanded it can cope with six S100 boards. On board expansion is supplied as various “Levels”. The UK distributors will supply each level separately but do a “special” consisting of levels A, B and D. The actual constituents of each level and what happened to level C will become clear later. A further mention of the board is worthwhile because all the spare space is filled with little perforations – ideal for doing your own thing in.

Level headed

Although the Explorer is available built, tested, cased etc etc, I had the kit version consisting of levels A, B & D. Each level is complete with its own assembly manual and description. In general the assembly instructions were clear and concise. Construction is made easier by the provision of sockets for most of the ICs.

It is worth taking time over the construction as mistakes are more easily prevented than detected (thought for the month) and there are plenty of joints to make mistakes on. I did start to calculate how many solder joints I had made but it made my brain hurt, so suffice it to say it’s a lot. Despite this the only real problem I encountered was that the silk screened component positions were sometimes ambiguous. Rather than tossing a coin I used the circuit diagram provided to clear up any doubts as to what went where. Also in working from the assembly drawing I noticed that to astound and confuse presumably R144 had been omitted. I eventually found the position using the legends on the board. Although the UK distributors will rescue constructors with problems anyone in doubt about their ability to build the kit would be wise to splash out the extra cash on a ready-built version.


Close-up of the main board showing the empty RAM and ROM area and the S100 expansion slot.

Level A provides 2K monitor, 256 bytes of RAM four 8 bit and one 6 bit I/O port, 14 bit clock/counter and interfaces to RS232 or 20 mA terminal, cassette and printer. No buffers are provided for the latter but spaces for suitable devices are present. A remote switch for the cassette recorder motor is on board. A version with monitor ROM to operate a Hex keyboard and display is available. A user interrupt and reset switch are also incorporated on the board.

Level B is the buffer/driver/address decoder hardware for both the on board expansion (levels D & E) and up to six S100 boards.

Level D is 4K of RAM in the form of 8 2114’s. This can be located anywhere in the address space 0000 to EFFF. F000 to FFFF being used by the system ROM.

Level E consists of sockets and voltage regulator to enable 8K of 2716’s to be installed. If required these EPROMs may be replaced by pin compatible RAMs. It is a hardware feature of the Explorer that each expansion has its own voltage regulator to minimise any problems due to excessive drain or surges to any part of the system.


The main PCB cases with its power supply. Note the generous supply of socket holes on the back – good forward planning.

Above C Level

Further expansion is not possible on board so the only way to go is up. Pads for 2 S100 connectors are provided in which two boards can be mounted vertically. Alternatively the level C motherboard can be bolted on to the main board, picking up the bus from one of the connectors. This enables six S100 boards to be accommodated in total – five on the motherboard and one in the spare connector on the main board. I understand that Newtronics now have the North Star Disc system operational for the Explorer.


The construction details given in the manuals are good. They start with a description of what the plastic bag full of bits is going to do. There is then a check list of parts down to the last nut and bolt. Resistor colour codes as well as their values are given in this section. The actual constructional instructions are clear, concise and follow simple short steps. This means that by ticking off each instruction as it’s completed, disastrous errors caused by distractions and interruptions can be averted.

The manuals contain a minimum of extra details but the level A instructions include details of how to use the monitor commands and gives an example of how an idea gets to be a program and how a program makes things happen.

The example requires the connection of a loudspeaker to an output port which is made to produce a 1kHz tone for 5 seconds.

Overall I found the package was let down by the presentation of the documentation and scarcity of extra (although some may say unnecessary) information, for example block diagrams and I/O implementation.

I notice that an Intel 8085A users manual is available from the distributors and this would probably be worth considering in view of the dearth of information provided.


The monitor resides in 2K of ROM from F000. Two versions are available, one for RS232 or 2 mA peripherals and one to run from a Hex keypad/display. As the RS232 version is the most complete. I intend to ignore the Hex version.

Full details of the commands are given in Table 1. There is not much of special interest here but a few points are worth a mention.

The terminal width can be set by altering location F8FA or F8F6 – the manual seems a little confused here (F8FA gets my vote as it’s mentioned 3 times as opposed to F8F6 only once.)

Sending a “space” from the terminal to the machine automatically sets the Explorer to the terminal’s baud rate.

The tape load and save commands are straightforward enough but as the manuals warn some juggling with controls could be necessary to get good loads first time. On loading a parity check of each byte is carried out and a failure causes the cut off of the tape’s motor and an error message to be printed. This either means that you are trying to load “Semprini and his Silver Strings play The Sex Pistols’ greatest Hits” or you’ve got a problem somewhere along the line. Obvious candidates are magnetised, or dirty head, shoddy tape or simply that the program was not saved correctly. No verify command is available in the monitor so it would be wise to set up the tape and make sure it’s working before keying in that 56K “Star Trek Meets the Supreme Commander” program.

A useful addition to the tape routines is the ability to give programs headers when recording them and search when loading.

Inputting any character other than 0-9 or A-F when the machine is looking for a hex number results in the error symbol (*) being displayed. The monitor does not provide for breakpoints but this is a minor grumble over what appears to be an adequate monitor.

Wares Of The Soft Kind

The 8085A is completely software compatible with the 8080, the real difference being in the speed and the addition on the 8085 of serial ports and the instructions to handle them, so all your favourite 8080 programs can be run on the Explorer. As previously mentioned a North Star disc system can be hooked up to the Explorer opening up the realms of CP/M.

Microsoft 8K BASIC 80 is available in ROM or on cassette for the Explorer but as there have been so many accounts of the language in CT and elsewhere I won’t bore the devotees or antagonise the critics of this language by going through it all again. Suffice it to say that all the normal functions are included.


No keyboard is supplied with the Explorer, neither does it have the capability to directly drive a display. This means that unless you’ve got the odd terminal kicking around doing nothing you and your Explorer are going to suffer severe communication problems. Fear not – Newtronics to the rescue! They sell a terminal (ASCII keyboard and video board) which could have been made for the job. The display can go to either a TV or monitor. The character font consists of the full ASCII character set plus Greek (yes Greek) alphabet. Cursor control is also available, just add a PSU and you’ve got all you need.

For those among us who consider video display frivolous and would rather produce pictures of scantily clad young ladies on a Teletype, the Explorer can help you too. Interfacing with a Teletype is simplicity itself and probably one of the cheapest ways to produce hard copy.

A case to house the Explorer and a power supply to run the full expansion is also available. There’s even a cut out for a fan but I’m told the machine in the shop stays cool without one.


The Explorer is definitely a quality basis for a full system, as large as you need. Unfortunately, the price reflects this. With only levels A, B & D, a Newtronics terminal and a power supply, it sets you back around £500. Viewed in the context of a system being readily expandable (as this definitely is) the cost is not prohibitive but whether it represents value for money would very much depend on the individual’s immediate application and future plans for the system.


Thanks to HL Audio (Newtronics) for the loan of the machine and their help in preparing this report.

Table 1. The monitor commands that are available.
Explorer 85 Monitor Commands
Command Name Instruction Format Explanation
CHANGE C(location) Modifies contents of memory location. Pressing space bar causes Explorer to display contents of memory specified and it can be replaced by typing byte in Hex. This followed by space bar causes next location to be displayed. Carriage Return (C.R.) terminates instruction.
FILL F(from),(to),(byte) Writes (byte) specified to block of memory given.
DISPLAY D(form),(to) Displays contents of specified locations. Either 8 or 16 locations per line of display depending on terminal width.
MOVE M(from),(to),(new start) Writes specified block of memory to block starting at specified address.
INSERT I(from) Allows for insertion of strings of data each byte separated by comma, space, CR or solidus. ESC or non-Hex character exits from command.
EXAMINE X(reg code)


Displays contents of specified register.

Examine contents of all registers

STEP S(from)


Single step from specified instruction.

Single step from instruction indicated by Program Counter. Space bar causes next step to be executed. CR causes next step to be executed and register contents displayed.

GO G(from)


Execute program from specified address.

Execute program from address specified by PC.

RECORD R(from),(to),(byte) Dumps to tape contents of specified block giving it the header (byte).
LOAD L(byte) Searches for program (byte) and loads it with parity check.
Note: Address are all 2 byte hex Nos.
Reg Codes:
  • A = Accumulator B to E and H & L the appropriate register
  • F = Processor flags
  • I = Interrupt status and mask
  • S = Stack pointer
  • P = Program counter

First published in Computing Today magazine, June 1980


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