Hi,
how can i communicate my HP48GX with an Motorola HC908KX8 microprocessor by RS232 SCI?
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HP48 - Motorola MC68HC08 SCI
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07-11-2004, 10:16 PM
Hi,
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07-12-2004, 01:11 PM
Hi Mauricio... HC08s are nice little CPUs! :) Which variation are you using, how much ROM & RAM? What else does this CPU have to do beside communicate with the HP48? I'm not an HP48 owner/user but have done a lot of serial comm work for other embedded widgets. I seem to recall the HP48 does (sometimes? always?) use a form of the Kermit transfer protocol for transfers. I'm not sure if 48 user programming can do byte-by-byte serial communications. I also don't recall the HP48 bitrates/ parity/ stopbits anymore - or even if it configurable by a user program - but when I looked years ago I recall it was something standard. The HP48 serial port, IIRC, is at a logic level (+5VDC, swinging down to 0V) -- whereas 'true' RS232 communication requires a level converter to deal with swings between +9 to +12VDC and -9 to -12VDC. So you can connect the 'HC08 to the HP48 directly without an intervening level converter device (like Maxim's MAX232) if your 68HC08 is a 5 Volt CPU. A 3.3V 68HC08 CPU might be problematic but might work. Just set up your SCI port on the HC08 for the proper bitrate, parity, stop bits. Do not use hardware RTS/CTS handshaking. If the 48 can only communicate using Kermit protocol, and if you're using a C compiler on the 08, you should be able to get Kermit open source code and 'trim the fat' (i.e, remove useless features') to keep code size down to what you can fit in your HC08.
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07-12-2004, 02:15 PM
Quote: You might get away electrically with connecting it directly, but it won't work unless you bit-bang the UART rather than using the hardware UART. EIA-232 drivers and receivers are inverting, so if you connect it directly you'll have the wrong polarity. I highly recommend using a MAX232 or one of the many equivalent parts. That way you can hook the device up to any EIA-232 devices without risking damage, and you have better ESD protection than using a port pin directly. If you use some kind of evaluation board, it's likely that there's already a level converter present. ▼
07-12-2004, 06:04 PM
Hi Eric... All I was saying was _if_ the HP48 was running 5V serial w/o a MAX232 he could directly connect it to a 5V 'HC08, and that maybe a 3.3V HC08 could work too. No inversions, etc. required...
Bill W ▼
07-12-2004, 10:01 PM
Quote: But the inversion IS required, if you want to use the hardware UART. UARTs have logic levels that are high for space, low for mark. But EIA-232 (even the non-really-EIA-232 of the HP-48) is negative (or in the case of the HP-48, not very positive) for space, and positive (on the HP-48 more positive) for mark.
Example of the transmission of an uppercase letter "E"
+2V ____ ____ ____ ____ ____ So if you hook up the HP-48 directly to the UART of a 5V 'HC08, it isn't going to break the 'HC08 (or the HP-48), but it also isn't going to work correctly. If you implement a software (bit-banged) UART, you can obviously deal with the inversion in software.
07-13-2004, 04:42 PM
Not true! The HP48 does use "true" RS-232 voltage at +/-4.3V, even if they are not the 12V you're used to getting out of a PC's serial port. Quoting Preston Brown from the comp.sys.hp48 FAQ: Quote:
07-16-2004, 12:37 PM
Quote: You recall incorrectly. Maybe you were thinking of a Sharp model?
For transmitting, it's (minimum) +3.0V for a space and -3.0V for a
For receiving, it's +1.0V to +15.0V for a space, and -15.0V to The absolute maximums are listed as -25V to +25V.
Quote:
Actually, if I recall correctly, RS-232 is supposed to transmit at
So, although it doesn't fully comply with RS-232, it's generally
Information about the 48 series I/O is available at
Quote:
The 48 series can also use "raw" serial I/O. That is, without any
Actually, the printing commands (with flag -34 set, "Print via
The 48G series also has the Xmodem protocol built-in, and an
The built-in speeds on the 48 series are 1200, 2400, 4800, and
But "Serial IR" is 2400 bps only, and is half-duplex instead of
The 49G series is similar to the 48G series for I/O via wire, but
Regards, |