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Speed tickets on RS232 Level Converters

As practice has shown, simple level converter with -12V supply derived from DTR/RTS is not capable to work on high speed. Therefore I have made several tests on speed with the original PNP-based design and a RS232 line driver SN75150 and publish test results in this post. Test summary is given in Table 1.


Experiments were made with four RS232 Level Converter designs:

  1. PNP Transistor, parasitic -12V
  2. PNP Transistor + JFET current source, parasitic -12V
  3. PNP Transistor, external -5V supply
  4. SN75150 – Dual RS232 Line Driver

Test environment:

  • TTL output generated by SISAM in diagnostic mode
  • Converter loaded with 1.5m shielded cable (C?‰?100-300pF) + PC USART (Winbond)

Data verification methods:

  • watching and reporting framing error condition and
  • comparing received data to expected

Test tools:

  • SITEST ??“ sends commands to SISAM and forwards data to stdout
  • LOOFI ??“ reads data from stdin and verifies the stream against the master pattern, made of the first count characters.

Command lines for tests are the following

SITEST /port=COM1 /count=640000 U03; > c:tempU03.txt
LOOFI /count=224 /file=c:tempU03.txt

SITEST sends U03 command to SISAM. This command turns SISAM in diagnostic mode and SISAM sends sequences of characters 0x20 to 0xFF using baudrate, specified in the command (U00 ??“ 115.2K, U03 ??“ 921.6K). SITEST saves data to file and aborts execution if a framing error is detected. Then LOOFI is used to verify data.

1. Hardware loop test

In this test RX and TX wires were connected on the free cable end and test data were send with a terminal application. Test has passed 460.8K and has failed on 921.6K ??“ wrong data received.

2. PNP parasitic -12V

Test has passed 230.4K and has failed on 460.8K with farming error. Oscilloscope has shown that falling edge is too sloping. I have tried to lower R1 and R2 and test has failed. I also tried to shorten the cable from 1.5 m to 0.3m and it did not make results any better.

RS232 Test 2

3. PNP+JFET parasitic -12V

As a variation of previous design, I have replaced R2 with a 10mA current source made on BF245A. This did not make results any better. My explanation to this ??“ parasitic power is not a good power supply to drive even small capacitive load at that high speed.

RS232 Test 3

4. PNP external -5V

In this test the circuit was powered with??“5V external power supply. R1 and R2 were varied from 1K to 100 Ohm. The best results were achieved with speed R2=360 and R1=10K – Test has passed 460.8K and has failed on 921.6K with framing error.
These resistor values cannot be used with parasitic power, because transistor’s collector current (Ic?‰?30 mA) is higher than can be derived from a single COM port line.

RS232 Test 4

5. PNP+JFET external -5V

This design I did not try because it needs another type of JFET with IDSS current 30mA and I did not have it handy.

6. SN75150

SN75150 is a dual supply dual RS232 line driver. Despite SN75150 datasheet states maximal baud rate 120K, it works on 921.6K and the test has passed 921.6K.

I tried to power it with +5V and -12V parasitic power and it did not work at all.

RS232 Test 5

7. Summary on tests

Table 1

Test Max baud rate
Hardware loop 460.8K
PNP parasitic -12V 230.4K
PNP parasitic -12V, 0.3m cable 230.4K
PNP+JFET parasitic -12V 230.4K
PNP external -5V 460.8K
PNP+JFET external -5V N/T
SN75150 921.6K
SN75C3232 N/T
MAX3232 N/T
MAX232 N/T

N/T – Not tested

6. Notes on driver selection

In this chapter I provide my motivation for driver selection.

I have looked for drivers in TI Interface Selection Guide. None of 1000 kbps drivers were available from local dealers (besides those drivers are not cheap). Therefore I decided to take a 120K driver and ‘overclock’ it. Considering that the current level is important for driving capacitive load, I limited my selection to dual supply chips. With these constraints I picked the smallest PDIP chip available locally and it happens to be SN75150. Its advantages: small footprint, very little overhead, level compatibility with PNP-based design, low price. Disadvantages ??“ necessity of dual power supply.

If dual power supply is not acceptable, you may try other drivers (such as SN75C3232, MAX3232, MAX232). I would appreciate if you share your results with me.

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