Pin 5 of IC11 should be bent out from the socket and the connections made
with an insulated “soldercon” socket. Increase the potentiometer value
until the flash just disappears.
We don’t guarantee the results, but we’ve tried it and it seems to work!
Due to an artwork error, pin 35 of the UART (parity inhibit) has been left
floating. It should be tied to +5V, and this can be done by adding a small
link on the solder side of the board between pins 34 and 35.
In practice we have found that the absence of a connection to this pin
generally makes no difference, but it can occasionally cause intermittent
cassette loading or incompatability between systems. In certain circumstances
it may be possible to increase the data transfer rate of the cassette load and
dump. This can be done by increasing the frequency to pins 17 and 40 of the
UART (IC29). The standard frequency at this point is 3.9 KHz, which is taken
from IC2 pin 11. The transfer rate may be doubled by taking the UART clock
from IC2 pin 12 (7.8 KHz) or quadrupled by taking the UART clock from IC2 pin
With each increase of speed, however, the record replay levels and the quality
of the tape and recorder used, and variations between records become much more
critical. Using a £40 portable cassette recorder and good quality tapes we
have been achieving a 100% load rate at 4 times the standard speed.
If fitting this modification the option of selecting between the normal and
fast speeds should be available. This could be done by feeding the higher
frequency into Pin 1 on the Nascom (marked EXT SERIAL CLOCK) and then selecting
between speeds by means of a switch at link 4.
It is recommended that cassette interchange between users should always be done
with cassettes recorded at the standard speed. The higher transfer rate should
only be used for loading and dumping the users own programs.
We have found that the tolerance of some manufacturers components can lead to
problems with the timing of IC18 (74LS123). The problem manifests itself when a
character with bit 6 set (e.g. any capital letter) is followed by a character
with bit 6 clear (e.g. any number). If these are displayed on the screen, the
first character will disappear!
The problem can usually be alleviated by (i) interchanging I.C.s 7 & 18 (ii)
cutting off either leg 5 or 12 of I.C.18 and then putting a link across the
solder side of the board between pins 5 & 12. (iii) changing the device for
that of another manufacturer.
We have had a number of enquiries about interfacing teletypes and other terminals
to the Nascom 1. Both 20 mA loop and V24/RS232 serial interfaces are brought
to Sk2. We suggest you make up a cable bringing these signals to a Cannon D
type socket with the following pin allocations so that it is easy to change
terminals or swap between systems.