Replacing BAT85 with BAT54

[Andres Rodriguez]

Reading both datasheets they seem to have identical electrical characteristics (or I missed something again). Would it be OK to use the BAT54 in the enhanced download circuit?
I have several Bat54 in SOD323 package and if I could use them it would solve a space problem in my PCB.

 

[westaust55]

Electrically they are virtually one and the same. Go for it.

 

[BeanieBots]

Just about any schottky will do the job but it must be a schottky type.
The purpose is to clamp the negative input voltage from RS232 to less than -0.3v. A 'regular' diode does not conduct until about 0.6v and so won't do the job.
BAT54 will be fine.

 

[hippy]

I've used a standard diode in series to block negative voltages which seems to have worked for me ...

     1N4148       ___
RX >---|>|---.---|___|---> PICAXE
            .|.   22K
            | |
        10K |_|
            _|_ 0V

 

[Mycroft2152]

I've used a standard diode in series to block negative voltages which seems to have worked for me ...

     1N4148       ___
RX >---|>|---.---|___|---> PICAXE
            .|.   22K
            | |
        10K |_|
            _|_ 0V

I would be very careful when using the above circuit with laptops. Many laptops do not provide the "official" RS-232 voltage levels. Adding a 0.6 volt voltage drop from the diode could cause downloading problems. Some PICAXEs, like the 18X, are very sensitive to programming voltage levels.

Myc

 

[BeanieBots]

The circuit shown by Hippy is actually my prefered method when using +/-12v RS232 signals because it reduces the -ve drive to 0v as apposed to -0.3v. However, things can get a little 'close' when using a series diode if the signals are only 5v/0v such as is often found on many laptops. However, if you do have 5v/0v serial logic, then of course, you don't need this part of the enhanced download circuit at all.

Just pipped by Mycroft2152 re the 5v logic.

 

[Mycroft2152]

The circuit shown by Hippy is actually my prefered method when using +/-12v RS232 signals because it reduces the -ve drive to 0v as apposed to -0.3v. However, things can get a little 'close' when using a series diode if the signals are only 5v/0v such as is often found on many laptops. However, if you do have 5v/0v serial logic, then of course, you don't need this part of the enhanced download circuit at all.

Just pipped by Mycroft2152 re the 5v logic.

I found out about the 18x sensitivity the hard way. I had an old Thinkpad laptop that would program 08Ms perfectly. but souldn;t prgram an 18X. I ended up making a simple TTL inverter buffer adapter and it worked.

Myc