Chronology, time and dates and a review of the Gemini GM822
D. R. Hunt
As was mentioned in the last issue of 80BUS News, the series ‘Kiddies
Guide to Assembler Programming’ had found its inevitable end as the topics to be
covered were becoming too long and complex to be dealt with in a single issue.
However, as the urge to write in a, hopefully, understandable manner is no less
diminished, the search for other topics has continued. In this article the
intention is to cover the Gemini GM822 real time clock kit from both the
construction and the software point of view.
Of late a number of real time clocks have appeared for the Nascom and
Gemini, five at the last count, ranging in price from about £25.00 to £50.00,
with one at £125.00. The later is the Gemini GM816 I/O card which also contains a
CTC, three PIOs and fairly comprehensive expansion capability as well, which
explains the otherwise highish price. Of the cheaper clocks, these are small
cards measuring up to three inches square and are intended to be piggy backed
onto the host board with double sided sticky tape, or to be mounted separately as
convenient. Four of the five clocks feature the National Semiconductor MM58174A
chip, the fifth I have not seen, but from the price, it would be reasonable to
assume the same chip. All incorporate battery backup. The clocks are provided in
either built or kit form with the necessary software and documentation to build,
connect and run them.
The Gemini GM822 is the only version I have had experience with, and this
part of the article will be confined to a review of that item. The Gemini GM822
is supplied in kit form at £29.50 + VAT, a middle of the road price when compared
with its competitors.
The kit comprises the documentation and the component parts. The
documentation is extensive, covering the constructional details, a detailed
description of the circuit employed, some three pages on the workings of the
MM58174 itself which is considerably more understandable than the National
Semiconductor data sheet for the chip (not supplied), and a simple software
listing in both assembler and HEX dump for either the Nascom or Gemini machines.
The routines are simple and well explained. Because they are short, however, the
routines are unforgiving of input errors, but form a sound basis for more
elaborate routines which may be written.
The components were all there as per the parts list, although a
connecting lead to the PIO plug was not provided. The printed circuit board is a
little disappointing, and not up to the usual Gemini standard. The pcb is glass
fibre and is quite small, about 4cm by 6cm, the tracking is single sided only.
Unfortunately the board is not coated in the usual Gemini green solder resist and
it is only too easy to bridge pins of the IC sockets or the 26 way IDS connecting
plug during assembly. Extra care during assembly and inspection needs to be
taken. The only other problem encountered during assembly was with the tantalum
bead capacitor. The documentation blithely warns about ensuring that the
component is connected with the correct polarity, but as the type supplied was of
the colour coded variety with no polarity markings, this could present a problem
for those who do not know the correct connection. For those who do not know the
correct connection, the positive lead is on the right when holding the capacitor
leads down with the coloured spot facing you. No other problems were encountered
Two clocks were ultimately constructed, the first one at home and the
second at work a few days later. So, to the first clock, constructed and tested
at home. The software was typed in from the HEX listings provided, and checked.
The port base address, 04H for Nascom, or 0B4H for Gemini was changed throughout
to 1CH to suit a spare port on my Nascom I/O card, and to remain consistent with
some already existing software which had been acquired from others who already
had the clock working. The clock was connected with a 6" ribbon cable folded ‘Z’
shaped and flattened to shorten-it to about 2" and the clock stuck down on top of