DSP232+ and the RTS / CTS fix

If you own a Timewave DSP-232+ TNC and are trying to connect to it using a USB to serial adapter, then you have likely come across issues when trying to operate in KISS mode. No matter what the settings, it appears that the DSP-232+ requires hardware flow control using the RTS (ready to send) and CTS (clear to send) control lines.

RS232 pinouts

Most USB to serial adapters that I’ve worked with do not have the facility to use the RTS / CTS lines. One solution would be to replace the passive DB-9 serial plug with a DB9-USB-M which acts as a USB to serial converter that allegedly supports RTS / CTS.

DSP232+ RS232 schematic

In this case, I plan to short out RTS to CTS on the DSP232+ circuit board, fooling the DSP232+ into believing there is hardware flow control from the host. Looking on page 275 of the DSP232+ manual, the schematic for the RS232 port is shown.

Looking to make the hardware flow control fix a temporary measure, I would like to short out the RTS and CTS pins with a jumper wire. This is most easily accomplished by looking for convenient connection points in the circuit.

The RTS line (pin 7) and CTS line (pin 8) are shown as connecting to decoupling capacitors C11 and C15 respectively.

To short the RTS and CTS lines, we will look for the “high side” of the decoupling capacitors on the circuit board.

Location of C11 and C15 on DSP232+ circuit board

Looking at the PCB, the manufacturer was kind enough to label the components on the top-side. The red arrows in the accompanying image indicate the “high side” of the capacitors (note the thick ground-plane on the opposite side of the SMD caps?).

Using a thin piece of insulated wire, I will solder one leg to the capacitor C11 (lower right) and insert the other bare end into the via shown at C15.

C11 to C15 jumper soldered in to short RTS to CTS

With the jumper in place, we can test the DSP232+ with a USB to serial adapter, putting the TNC into KISS mode and waiting to see packets in the command line.

I will admit that my solder job at C11 is not the most beautiful sight to behold, but it will work for this test.

Notice also I’ve got some bodge wires – they connect the DB9 connector to their signal-appropriate nodes on the PCB. The old USB to serial style adapter which came with this DSP232+ was fried and the traces were lifted, necessitating a repair job to make the unit functional.

In other news, I finally received the DS1216C that I’ve been waiting to try as a RTC for the DSP232+. The Dallas parts never did work when I tried to parallel them with the EPROM U5 shown in the above image. The DSP232+ boots normally with the DS1216C stack as shown and so I used some hot-glue to hold everything in place as the DS1216C has less than stellar mounting to the original socket.