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Re: [midatlanticretro] RS232 Again?

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  • Dan Roganti
    ... The interweb has plenty of info out there to help with this. There really isn t any mystery to the interface. After a few min of searching for Seagate
    Message 1 of 12 , Jul 11 4:57 PM
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      On Thu, Jul 11, 2013 at 11:32 AM, RonK <rkushnier@...> wrote:
      I thought you guys could appreciate this.

      I've been trying to "unbrick" a Seagate SATA 750 GB barracuda hard drive. The recommended approach is to go through it's 4-pin jumper connector to reach its serial port. http://www.overclock.net/t/457286/seagate-bricked-firmware-drive-fix-with-pics/240#post_20364127

       It's "Back to the Basics" of RS232 technology.  Oh no!  Would I really have to open that trapdoor in my brain that I hadn't accessed for thirty years, and pull out all the old stuff? I guess so. Baud rate, Parity, STOP bits, Flow Control- oh all that wonderfulness... here again!


      The interweb has plenty of info out there to help with this. There really isn't any mystery to the interface. After a few min of searching for Seagate serial port, it clearly defines this to be a TTL level interface. Even the com settings are defined - just pop the settings into your terminal pgm and go. And the RS232 - TTL converters cost only a few bucks. 


       

      My main concern though, were the required voltage levels. It seemed that the Seagate drive wanted TTL levels. So it was recommended that the use of an RS232 to TTL adapter be employed. I didn't have one of those, but I did have a USB-to-RS232 adapter. I figured that unless the adapter employed some sort of voltage doubler to get the voltages up to the normal Rs-232 levels, I would be safe with the 5 volt USB power. Well that didn't work.  See my overclock post above for more details.


      This is where you can find some info about the Seagate serial port being only TTL and addt'l instruction to access the Seagate drive. 

      The fact that the MAX232 level converters are inverted is a non-issue. This is part of the RS232 standard. A Mark signal, negative level, is a logic "1' and vice versa - this conversion is done transparently in hardware. This is why it's important to use the correct adapter. And why you want a RS232 to TTL adapter  - convert RS232 levels to TTL levels and not the other way around. With the USB to RS232 adapter, you are converting TTL levels ( albeit differential on USB) to RS232 levels - which is the opposite. 

      As for the RS232 to TTL adapter, you can find them easily online for only $5 and free shipping, even on eB@y

      And you could even build one with only 2 transistors, a few resistors and a diode.

      Dan



    • Jeff Jonas
      ... I m not at home this week so I can t check what parts/boards/assemblies I have but there are many lovely choices - the Adafruit FTDI friend converts TTL
      Message 2 of 12 , Jul 12 1:01 AM
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        > This is where you can find some info about the
        > Seagate serial port being only TTL

        I'm not at home this week so I can't check what parts/boards/assemblies I have but there are many lovely choices

        - the Adafruit "FTDI friend" converts TTL serial to USB, so it naturally works with Linux, Windows, OS/X, etc. There are several variations of the FTDI chip board, the most important options being jumpers for power source, 3.3 or 5V serial, and how many signals are available (the FTDI chip offers a lot for handshaking and "bit banging")

        - there are several MAX232 chips from which to choose. The most basic converts 5V TTL to RS232 using 4 external capacitors. Others allow 3.3V logic and/or no external capacitors. For breadboarding and experimenting, I prefer the pre-assembled postage stamp size boards, available from many sources.

        --jeff Jonas
      • joshbensadon
        ... I recently bought 5 Maxim RS-232 chips from China, very cheap. But I got burned (literally). Those chips had some defects. They would get extremely hot
        Message 3 of 12 , Jul 12 6:27 AM
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          --- In midatlanticretro@yahoogroups.com, "Jeff Jonas" <jeff_s_jonas@...> wrote:
          >
          > - there are several MAX232 chips from which to choose. The most basic converts 5V TTL to RS232 using 4 external capacitors. Others allow 3.3V logic and/or no external capacitors. For breadboarding and experimenting, I prefer the pre-assembled postage stamp size boards, available from many sources.
          > --jeff Jonas

          I recently bought 5 Maxim RS-232 chips from China, very cheap. But I got burned (literally). Those chips had some defects. They would get extremely hot and pop. I tried some Intersil chips from my old (diminishing) stock pile, no problem.

          :)J
        • dfnr2
          ... The *-232A versions are easier to work with and require only 0.1uF caps. New chips from reputable vendors can be had for a little over $1:
          Message 4 of 12 , Jul 12 7:40 AM
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            --- In midatlanticretro@yahoogroups.com, joshbensadon <no_reply@...> wrote:

            > I recently bought 5 Maxim RS-232 chips from China, very cheap. But I got burned (literally). Those chips had some defects. They would get extremely hot and pop. I tried some Intersil chips from my old (diminishing) stock pile, no problem.
            >
            > :)J
            >

            The "*-232A" versions are easier to work with and require only 0.1uF caps. New chips from reputable vendors can be had for a little over $1:

            http://www.mouser.com/Semiconductors/Interface-ICs/RS-232-Interface-IC/_/N-45lx1Zscv7?P=1yztkj3Z1z0xwq2Z1yzrawtZ1z0y3waZ1z0z44dZ1z0y33tZ1z0y33vZ1yzspjfZ1z0xuzo&Keyword=232a&FS=True&Ns=Pricing|0

            Dave
          • Mike
            From time to time, those RS232 to TLL adapters come in handy. I use the MAX202 instead of MAX 232, as it is a little easier to deal with. This single sided
            Message 5 of 12 , Jul 12 8:07 AM
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              From time to time, those RS232 to TLL adapters come in handy. I use the MAX202 instead of MAX 232, as it is a little easier to deal with.

              This single sided PCB I designed for the SCELBI does essentially what you need, It does have an extra flipflop needed for the SCELBI TX interface that can be bypassed (leave off the 7474 and jumper pin 9 to 13).

              http://www.willegal.net/blog/?p=4459

              I originally did a version of the board without the flip flop, that I used to reprogram a Sony CRT a while back. That is probably exactly what you need, but I don't know where the artwork is, at the moment.

              One of my PS/2 to ascii keyboard adapters could also be adapted, by leaving off the micro-controller and connecting directly to where the TX and RX pins on the micro-controller would normally be.

              http://www.willegal.net/appleii/appleii-kb-int.htm

              If you are familiar with AVR programming, the PS/2 adapter could also be repurposed as a parallel to RS232 converter (paper tape reader?), but that is another story.

              regards,
              Mike W.


              --- In midatlanticretro@yahoogroups.com, joshbensadon <no_reply@...> wrote:
              >
              > --- In midatlanticretro@yahoogroups.com, "Jeff Jonas" <jeff_s_jonas@> wrote:
              > >
              > > - there are several MAX232 chips from which to choose. The most basic converts 5V TTL to RS232 using 4 external capacitors. Others allow 3.3V logic and/or no external capacitors. For breadboarding and experimenting, I prefer the pre-assembled postage stamp size boards, available from many sources.
              > > --jeff Jonas
              >
              > I recently bought 5 Maxim RS-232 chips from China, very cheap. But I got burned (literally). Those chips had some defects. They would get extremely hot and pop. I tried some Intersil chips from my old (diminishing) stock pile, no problem.
              >
              > :)J
              >
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