|Re: HP-42S I/O|
Message #9 Posted by Christoph Giesselink on 10 June 2007, 5:59 p.m.,
in response to message #1 by Gerry Schultz
Some aspects don't mention from the authors before.
Memory limitation to ~7000 bytes.
The 8KB and 32KB 8 bit static RAM chips have a quite similar pin layout. The PCB of the High-End Pioneers is prepared for the use of both chips. So it's possible to replace the RAM chip with the 32KB on all high end pioneers. 2nd part are the MMU mask fuses in the Lewis chips. These are programmed for a memory device at address #50000 with a size of 32KB on all High-End Pioneers.
But why only the HP42S can make use of this additional memory getting ~31500 bytes of free memory whereas on the HP17B or HP27S there's no difference?
The answer is very simple. Only the HP42S firmware contain code expecting more then the usual 8KB of RAM (history repeats). So we can say that HP thought about HP42S with more memory but never sold them.
Now to the limited I/O capabilities of the High-End Pioneers. Therefore we have to go back into Saturn history. The HP71B, the first calculator with the Saturn CPU, was build out of several modules, display driver, ROM, RAM, ... made the complete thing quite expensive. On the other hand we have had a powerful mobile CPU (Saturn) which was predestinated for further use. The HP18C used this CPU but now in connection with a more integrated customer chip (1LP2 Centipede) containing reset control, oscillator, RAM, display driver, timer, input/output, ... reducing the amount of necessary chips to build a calculator. But from history we can see that this was still too expensive too, because only the HP18C and HP28C used this hardware.
Now my short comment to "history repeats". The HP28C was sold with 2KB of RAM (from the two integrated Centipede chips). But Educalc sold versions with more than these 2KB of RAM. The HP28C hardware was prepared to add HP71B RAM module chips and the HP28C firmware was adding this additional found RAM to it's memory pool. I don't know if HP ever thought to sell memory upgraded HP28C by themselves, but HP added code to the firmware to exactly do this without firmware change.
The successors, the High-End Pioneers and Clamshell calculators, use the Lewis chip. In a very very simple view the Lewis chip is a combination of the Saturn CPU and the Centipede customer chip. From the I/O side the Lewis chip inherit the I/O capabilities of the Centipede chip, nothing more, nothing less.
This is in fact a single bidirectional Input/Output pin. On the HP42S and on all other calculators with the Lewis chip this hardware pin was used to drive the IR transmitter LED.
I verified this on a HP28S (also using two Lewis chips), where I removed the IR transmitter LED and connected a simple hardware to adjust the calculator logic level to RS232C level. The interface is mostly powered by the PC RS232C interface, the power consumption out of the calculator battery is < 4 uA. The rest for the RS232C communication between HP28S and PC is software.