80-Bus News

First a “Thank you” to those of you who wrote in with messages of sympathy. It was obviously very late at night when the Editor put the last magazine together, and “Aunt Alice” was the only title that surfaced through the alcohol fumes. [Ed. – do you prefer this title?]

Denizen of Hell

It is strange how things resurrect themselves. Recently I’ve been rung about IMPs and IMPRINT, and I’ve also seen a letter on the same topic. I gather somebody somewhere has bought the remains of all the Nascom IMP printer and is selling the circuit boards and printer mechanisms as scrap. The IMP was best described as an early low-cost printer whose quality, of output was poor compared to the current Japanese offerings. The mechanism used did not offer very good registration, and IMP printouts reproduced directly in magazines were easily recognisable by the drunken appearance of the columns. (The registration on the IMP I had was bad despite careful adjustment. The best I could do was to get it to line up at about every tenth column across the page – others may have fared better). However it did produce legible printout, which is far better than nothing! I trust current IMP owners will forgive the use of the past tense above.

The IMP was actually controlled by a Z80 microprocessor. The original IMP was sold with NASPRINT as the control program installed in the printer. Subsequently I wrote IMPRINT [1], a replacement control program for the IMP which offered enhanced features such as selectable unidirectional-bidirectional printing, and a graphics mode. IMPRINT was supplied in a 2716 EPROM as a plug-in replacement for NASPRINT. Installation was just a case of removing the cover of the IMP, (easier said than done!), carefully extracting the NASPRINT EPROM from its socket, and inserting IMPRINT in its place, taking care to maintain the same orientation of the EPROM.

Be rude & Interrupt

For those of you embarking on adventurous software/​hardware projects (Like trying to breathe life into the remains of an IMP), don’t overlook the capabilities of your computer. For example when I developed IMPRINT I intially used the Nascom 2 to check the performance of the software in a non-destructive manner. In the IMP the print-head solenoids are driven via an output port and transistor buffers. The print-head has a maximum permitted duty cycle, and if this is exceeded the result tends to be a dead print-head and smoking drive transistors! An error in the software could have easily lead to the end of the project, leaving me with a totally useless printer. The answer was to disconnect the drive to the print-head during the development to prevent this happening. – “But then you couldn’t see what was happening” I hear you cry – wrong, this is where the N2 comes in again. The output of the IMP’s print-head driving port was connected to the PIO on the Nascom, together with a strobe signal. Three programs were then written for the N2.

1. An interrupt-driven routine which read a character from the PIO and stored it in a buffer in memory.
2. A program to initialise (a), enable interrupts,​and then echo characters from the keyboard (or elsewhere) to the IMP.
3. A program to analyse and display the contents of the memory buffer.