samc@hpccc.HP.COM (Samuel Chau) (05/10/88)
Reddy, The HP-71B bus is a 4-bit multiplexed address/data/instruction bus with 20 bits of address, i.e. 1Meg nibbles, or 512K bytes total. This bus is available at each of the four front module ports. RAM or ROM modules plugged into this bus have their own program counters and stack pointers, and so each device 'knows' if it is being addressed by the CPU. The operating system firmware performs soft configuration for plugged-in RAM and ROM modules. This means that the actual address space occupied by a particular module is determined by the '71B OS code, and is dependent on what you've plugged into the machine. So in a way, the entire 512K is accessible at each module port. The 64K OS in ROM is, however, hardwired to the lowest 128K-nibbles of address. Also, the largest RAMs and ROMs I've seen for the '71B are 64K bytes in size (or, more correctly, 128K-nibs). HP's 4K RAM module does not have a battery built in, and will forget everything you put in it the moment you unplug the module from the '71B. It is, however, possible to install a flat 3V lithium battery inside the module to keeps its contents alive outside the machine. You will also need to put a Schottky diode in series with the battery to prevent the '71B from charging the battery when the module is plugged in. I have made such a modification to one of my 4K RAMs. I use it to exchange files with friends who don't have either the card reader or the HP-IL module. I hope this helps. Sam Chau ...hplabs!hpccc!samc (408) 447-0238
samc@hpccc.HP.COM (Samuel Chau) (05/11/88)
>HP's 4K RAM module does not have a battery built in, and will forget >everything you put in it the moment you unplug the module from the '71B. It >is, however, possible to install a flat 3V lithium battery inside the module >to keeps its contents alive outside the machine. You will also need to put >a Schottky diode in series with the battery to prevent the '71B from >charging the battery when the module is plugged in. I have made such a >modification to one of my 4K RAMs. I use it to exchange files with friends >who don't have either the card reader or the HP-IL module. Oops! I forgot to mention that the above battery hack was completely my own doing. HP does not offer such modifications. It is also important to remember that tampering with modules will void the HP warranty. HP Corvallis has an established policy of not servicing their line of calculators, handheld computers or related peripherals if user modifications of any sort have been done. In other words, if you've hacked up your HP hardware, don't expect HP to fix it if it breaks. I modified my RAM module because I had a special use for it, the warranty has long since expired, and I don't care if it dies on me. If you really want non-volatile RAM modules for the '71B, the cleanest solution (not the cheapest) is to get the 32K or 64K battery backed-up RAMs made by CMT. For unit quantities, EduCALC has reasonable prices. ***************************************************************** * DISCLAIMER * * The opinions expressed above are strictly my own, not those * * of Hewlett Packard Company. * ***************************************************************** Sam Chau ...hplabs!hpccc!samc (408) 447-0238 ----------