prindle@NADC.ARPA (Frank Prindle) (08/29/89)
Beware that there are two kinds of RS-232 interfaces available for the C64. The "cheap rip-off" type (e.g. the Supra Peak Modem Interface) use only TTL logic to invert the modem data and control lines, producing 0 and 4 volt outputs instead of standard RS-232 levels (-12 and +12 volt). The better interfaces (e.g. the Aprotek and Omnitronix) use either the 9VAC from the C64 or a MAX232 chip to derive true RS-232 operating voltages for the outputs. Some modems (i.e. those which autoconfigure their inputs and outputs) will NOT work with the TTL level type of interface (they just sit there waiting for a higher voltage to make it's appearance on some pin or other). An example of such a modem is the Popcom 1200 baud modem. This is not to discredit the TTL level type interfaces (indeed, I've been using a 2-transistor homebrew TTL interface with my Avatex modem for years), but simply to explain why some modems may not work with some interfaces. Since both types of interface are often available in the $30.00 price range, why not buy the "True RS-232 level" type if you're in the market. Sincerely, Frank Prindle Prindle@NADC.arpa
gary.farmaner@canremote.uucp (GARY FARMANER) (09/02/89)
Frank, Generally the very cheap interfaces (which refer to themselves as MODEM interfaces) are TTL, and only support a minimum number of RS-232 lines. (for example, the Supra interface doesn't support DTR, which makes it useless for BBS purposes). Those that full RS-232 interfaces, have propert RS-232 level conversion and support a full set of RS-232 lines. This is very important if you want to use an MNP modem, since they only work properly if the CTS line is supported (although a kludgy XON/XOFF is usually available). Gary --- * Via ProDoor 3.01R