Most of the thermal printers I developed were for very high volume manufacture so cost was a big issue. In one very low cost product we got rid of the expensive chip used to de-serialise, latch and strobe data. You are limited in the number of connections to the head by physical constraint and the size of currents needed to print. So in one 64 dot print head we had the dots connected in an 8 by 8 matrix which was driven straight from the micro via suitable drivers. As there were no diodes in the matrix, when you fire one column there are some parasitic resistance paths through non-energised dots which you would at first think makes this idea a non-starter. However, because the dot needs to exceed a minimum temperature to melt the thermal ink, it turns out these parasitic paths never get enough current to get hot enough to melt the ink so the method does work. As over half the price of the print head was in the chip the savings were considerable even accounting for the external drivers we had to use.

Memory was always an issue to. The client wanted as large a printable character set as possible but did not want the ROM size to get any bigger. So we developed several bit map data compression algorithms. It is quite easy to dream these up but the clever bit is to create ones you can do in next to no time on a cheap 8 bitter.

Ian