I've been having a lot of fun (mostly using other people's instructions) with some of those cheap bluetooth modules (via ebay specials) from China. I'm just getting my feet wet learning the actual 'design' aspects of electronics, however.

I'm wondering how to take full advantage of the CTS and RTS lines on these modules and if anything needs to be done to make them compatible with older devices expecting an RS232c standard. The bluetooth modules seem to conform to the UART standard and voltages (3.3v vs 5v)

I note in the specs they give you, that they use a modified darlington style circuit off the TX line, presumably to hop the 'high' voltage on the TX line up to the 5v that the RS232 side expects. What I'm wondering is if I need to do something similar on the CTS and RTS lines if I wish to use those as well? One of the devices I am trying to connect to claims in it's instructions to use both of these connections.


  • Rx and Tx are 5V TTL logic compatible.
  • UART_TXD and RXD uses 3.3V CMOS logic compatible.
  • TXD and RXD are low swing RS232 bipolar compatible ( lower since doubled from 3.3V Vcc rather than 5V Vcc. so ~ +/- 4V swing)

You need to ensure compatiblity with CTS/RTS 3.3V CMOS and ensure polarity is correct for CTS/RTS TTL.

TTL threshold is 1.2V HCMOS threhold is is Vcc/2 or 1.6V so close enough and should be able to drive 1 TTL load. CMOS input protection over Vcc is all that is needed from 5V TTL. (series 1K and Shottky diode clamp to 3.3V)

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  • \$\begingroup\$ Thanks for the comment. After a little more searching I'm not finding that there's a DTR connector that they say should be set high. What I'm looking to do is to try to drop the bluetooth module into an older multimeter with an RS232 connector that has TX, RX, RTS, DTR and ground as it's only connectors. I'm not sure if they are using the DTR to let the meter know it's connected to the PC or to let the PC know it's connected to the meter. But assuming it's the former, what's the easiest way to set the DTR to high? \$\endgroup\$ – Scott Nov 20 '12 at 19:50
  • \$\begingroup\$ Unfortunately I'm not seeing any consistent 'high-when-connected' type output on the bluetooth module. (there's a pin for an LED to show connected status, but it 'blinks' when not connected, going solid when connected) \$\endgroup\$ – Scott Nov 20 '12 at 19:50
  • \$\begingroup\$ By the way, I'm probably going to pick up a MAX3232cse module to convert the UART to RS232 as it's smaller than I could make using the two transistors anyway and I'm working in a cramped space inside the meter. But that still leaves the DTR (and RTS), if in fact I need to do anything with them at all. sigrok.org/wiki/MASTECH_MAS345 \$\endgroup\$ – Scott Nov 20 '12 at 19:56
  • \$\begingroup\$ I also found this comment on another thread: MAS345 data format: 7n2 at 600 baud (7 bits, no parity, 2 stop bits). Control lines: DTR and RTS lines are used to power the TX line: RTS is clear for -12 supply; DTR is set for +12 supply. Data transmission is solicited sending whatever character to the RX line. \$\endgroup\$ – Scott Nov 20 '12 at 19:58
  • \$\begingroup\$ The device will have a high level out somewhere and then you can loop that back to DTR in the connector. It appears RTS and CTS are floating on the HC06 chip \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Nov 20 '12 at 22:04

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