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Transistor interfaces revisited

 
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mathdon
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Joined: 22 Jul 2008
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Location: Cambridge, UK

                    
PostPosted: Mon Aug 23, 2021 12:52 pm    Post subject: Transistor interfaces revisited Reply with quote

As I mentioned elsewhere, in this post, I recently bought a complete starter kit for the Arduino. I found to my surprise that it happens also to include everything needed to build the JP1.1 and JP1.2/3 transistor interface designs from Tommy Tyler. I know that these are totally obsolete, but the jp12serial library claims still to support them and I had no idea whether or not the many changes to that library code over recent years had conflicted with that support. So I decided to build them and find out.

As I don't still have a PC with a serial port, I bought this USB-to-RS232 serial converter which is a proper one that converts the voltage levels from TTL to RS232 levels. This is a report on my findings.

The JP1.2/3 interface works perfectly with all remotes of JP1.2 and later. I have done a test download from remotes with all the following processors: HCS08, S3F80 (JP1.3 and JP1.4), MAXQ610, MAXQ612, TI2541 and GP541. By connecting pins 3 and 5 of the 6-pin connector together and using the latest jp12serial, it also downloads JP1.1 but leaves the remote in communication mode, needing the batteries removed and replaced to restore normal operation. This is an inevitable consequence of connecting pins 3 and 5 together and cannot be avoided. This interface also works with a JP1 EEPROM adapter to download JP1 remotes.

I was unable to get the JP1.1 design to work as given in the link above. I traced this to the Rx output voltage from the interface to the converter being too low for the converter to recognize. I noticed that the resistors in the JP1.2/3 design have very different values from those in the JP1.1 design for otherwise identical circuits, so I changed those in the common ciruits to the JP1.2/3 values and then it worked properly. Note that despite this design having DTR rather than RTS connected to pin 2, it is used with the interface in RMIR set to JP1.X Serial and not to the newer JP11USB setting that is used for USB cables that have pin 2 connected to DTR.

Testing did, however, show up an issue with jp12serial. A JP1.1 remote used with this interface is identified as a JP1.3 remote. As a result, it works normally but is left in communication mode rather than restored to normal operation. I have fixed this for the next release of jp12serial.

Although Tommy says clearly that the JP1.1 design is not compatible with JP1.2 and JP1.3 remotes, jp12serial contains code for this design to work with these remotes. There is a problem with JP1.3 remotes in that code added for JP1.4/JP2 remotes, when used with this interface, sends data while pin 5 is high, which can potentially corrupt a JP1.3 remote. I have changed the testing order for the next jp12serial release to avoid this, so that the JP1.1 design will work with all of JP1.1, JP1.2 and JP1.3 remotes (and without needing to break the connection to pin 5 for JP1.3 remotes, which historically was necessary). The JP1.1 design should not, however, be used with JP1.4 and later remotes as jp12serial was never intended to be used with that combination.

I know that all this is of no more than academic interest, as I doubt that any users still have a working transistor serial interface, but the work has satisfied two needs of mine. I have always been curious as to the working of the JP1.1 design, which uses a capacitor/resistor combination for pulse generation, and I have now seen it in action, viewed with a logic analyzer. And I have been able to make sure that jp12serial still does what it was intended to do with these designs, even if they are obsolete, which gives me intellectual satisfaction Smile .
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