The Saturn (the processor inside the HP48) has a 4-BIT external data bus. This is a bad design, speedwise. This is because characters that are typed into the HP48 are 8-BIT, and decoding is required. I am not sure about the HP48's internal RAM/ROM, but I believe they are 8-BIT, and this requires more decoding to be used. It may be that there is a decoder that translates the Saturn's 4-BIT bus into an 8-BIT bus. This kind of decoder would be called a multiplexer and demultiplexer.
The HP48 has a 23-BIT address bus capable of addressing a 8MB memory map. This is not very useful, considering the only way you can use 4MB cards is if the card is personally made. No companies seem to sell them. You're probably thinking, "why worry about a 4MB card with a 8MB possibility?". Not all 8MB can be used. The HP48's memory map contains everything inside of the 48. The 256K/512K ROM (HP48S/SX, G/GX respectivily) has to be in the memory map at all times. Plus, the maximum memory you can put in both card slots (GX) is 4.125MB. The second card slot will accept a 4MB card, and the first will accept a 128K card. Not all this information can be accessed at once even. The HP48 uses a technique called bank-switching to address 128KB of RAM at a time.
The HP48's ROM was written well, but there are some things that should be changed. For example, the ROM can only transfer data through the serial port at up to 9600bps. But, using a software solution, it can transfer up to 19200bps (?) and even faster than that with no error checking. The same is with the IR port on the 48. Using the ROM routines, it can transfer at 2400bps, but using a software solution enables it to transfer at up to 7600bps with error checking. I am not sure which resistor you change to which value, but there is a resistor you can change inside of the 48 that will increase IR range and increase power loss as a side effect. The HP48 has Kermit and XModem written into the ROM. The HP48 can be used as an RS-232 device.
Absolute maximum UART speed with error checking and no error checking.
I do not speak for HP when I post here. This data was collected from personal experiences, and opinions.
More information will be posted here as it is gathered. This is Revision 2 of this document.
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Last Updated 16-Jan-98 ty140@hotmail.com