The circuitry for the battery backup is almost the same as used in the Mini-Memory module.
R1 has a little higher value and R4 a little lower value to make sure that transistor Q2
is switched on in all kinds of variaties like: original PEB and original RS232 card or
PEB with switched power supply and original RS232 card or PEB with switched power
supply and RS232 card without voltage regulators.
I used a battery-clip so that an empty battery can be easily replaced by a fresh
A few facts
- When this PCB is added to the RS232 peripheral card the top of the metal clamp-shell
does not fit any more. Some cooling elements for the voltage regulators can
be added in some way unless a PC-power supply is used for the Peripheral-Box.
- Serial port setting is 38400 Baud, 8 data bits, 1 stop bits, even parity and
no flow control.
- An uninitialized TMS9902 UART immediately generates an interrupt which
makes it impossible to load the DSR. The interrupt line must be cut or
disabled in some way. In this design a switch is added to disable
interrupts of the RS232 card if no DSR is loaded yet.
- The maximum DSR size is 4Kb. The whole >5000->5FFF area is used for
the parallel port. The pages 0 and 1 are selected by using the now unused
CRU bit 4 of the RS232 peripheral card.
- The final DSR will use two banks of 4Kb each. The lower half will contain
the original RS232 DSR without RS232/2 and the upper part will contain the
HDX1 DSR and file buffers for this.
- The TI is the master of the bus. The PC acts as a server and never send
- There is some hardware signaling build in but not fully used at the PC
side because it seems that the COM-port buffer can hold a lot of data,
but it works as follows:
The DSR (Data Set Ready) signal gives problems because that signal is tied to +5V on
the TI-side. Must see if this can be changed for RS232/2.
- PEB power on -> DTR on
- TI sending data -> RTS on
- TI reading data -> RTS off
- TI99HDX program not running or PC not connected -> DSR off
- There is some sort of error recovery in the HDX1 DSR. Every message
over the line haves a checksum. If the TI or the PC notices something
wrong it will ask the other side to re send the message.
- There is no way to write protect the RAM without major changes to the
original RS232 card design. I do not want to make major changes because
that will make this project to difficult. So there is always a change
that the DSR gets overwritten after some obscure system crash.
- There is no way the switch the interrupt line on/off without major changes
to the original RS232 card design. See above.
- The signal (names) of the RS232 connector are very confusing me. Seems
that TI gave it the name of the signal of the other end that should be
connected here. If you look at the schematic you see other signal names.
I will try to stick to that.
WARNING: Do not start this if you are not absolutely sure that you are
able do this.
Many steps to complete succes or total disaster
- You have been warned!
- Cut the interrupt line. The most simple location is at the trace of U6, pin #3.
Unfortunately this has to be done because it seems that an uninitialized
TMS9902 immediately generates interrupts which makes it impossible to load
a new DSR.
- Remove resistor R19 (4K7). This resistor pulls the DTR signal of
RS232/2 to 5V. We need this signal to determine if Ti99Hdx on the
PC is running.
- Remove the PROM U1. This is the most difficult part.
Gently cut all the 24 pins in such a way that
you're still able to solder the prom on a 24 pin socket so that you
can still go back to the original RS232 card.
- Heat up your soldering iron and gently remove the left overs of the
prom pins. Be careful not overheat the soldering pads.
- Place a 24 pin socket on the place of the just removed PROM. Mind the
orientation of the socket. Do not make the same mistake I did!
- To check that everything is still in working order you can place the
original PROM back if it is soldered on a 24 pin socket. Mind the
orientation! Check, check and double check!
- Concratulations if your RS232 card still works. Time to take a beer now
and leave the rest for another day.
- Now create the PCB
- Start with cutting the traces as shown in the pictures.
Before you start to cut check, check and double check!
- Add the wires and check, check and double check!
- Add the IC socket and mind the orientation and watch out for shorts
and check, check and double check!
- Add the print connector. This one is soldered on the bottom side. Make
sure that there are no shortcuts.
- Add the diodes and mind the orientation and check, check and double check!
- Add the resistors never mind the orientation but mind their place and check, check and double check!
- Add the transistor Q1 and mind the orientation and its place and check, check and double check!
- Add the transistor Q2 and mind the orientation and its place and check, check and double check!
- Add the battery clip and mind the orientation and check, check and double check!
- Enough for today. Take a rest and a beer.
- Check the PCB for shorts, loose contacts etc etc.
- Can't say it enough but ... check, check and double check.
- Now place the battery in the battery clip and measure the Voltage
between pin 14 and 28 of the RAM socket. You haven't placed the RAM chip
hey .. I did not tell you to do that yet!
- You should measure a voltage between 2.40V - 2.90V
- Remove the battery.
- Place the RAM chip in its socket. Mind he orientation and (all together know) check, check and double check.
- Place the PCB board in the 24 pin socket on the RS232 card.
- Connect a wire from address line A12 of the DSR RAM pin #2 to CRU4 (U12, pin #9).
- Connect a wire from the CE input of the DSR RAM to the DSRROM signal (U16, pin #9).
- Connect a wire from the WE input of the DSR RAM to the WE signal (U15, pin #9).
- If necessary, add a switch for enabling interrupts. Leave this switch in the
off position. You do not need the interrupts of the RS232 card if you never
use communication programs like TELCO etc.
- Place the RS232 card in the PE-BOX and switch the PE-BOX on. Your system
should not crash, make funny noises, generate strange smells or making smoke signals.
- Try to load the DSR with the supplied Ti99Hdx loader. If no error messages
are given then the PIO port and RS232/1 port should function. Remember ...
if the PE-BOX is switched of at this moment the HDX DSR is gone.
- Place the battery in the battery clip.
- Place the RS232 card back in the PE-Box.
- Reload the DSR with the supplied Ti99Hdx DSR loader.
- Enjoy ...