Electronic – Multiplexing TTL serial lines with transistors

serialttl

A master controller talks to multiple slave devices using TTL async serial. Instead of implementing software broadcast codes, slave select codes, etc, I want the slave select to be wired.

I use a shift register (instead of the pins in the circuit), and so I only need 5 lines from the microcontroller to control as many slaves I want.

The attached circuit is tested and works, but is it a valid design? How stable will this be? Am I wrong in assuming the current flows from TX to RX? In that case, what kind of switches could I use? Optocouplers?

TTL circuit

Best Answer

What you want is a digital multiplexer, in this case a 74LS157. The following schematic should give you an idea of how it would work. This would take 3/4 of a single chip.

schematic

^{simulate this circuit – Schematic created using CircuitLab}

In this case, a single bit is used to determine which slave link is active - 0 for Slave 1 and 1 for Slave 2. The only possible change you would need is (perhaps), to set the unused TX multiplexer channels to 1 instead of 0.

With readily available chips, you can increase the number of channels in powers of two up to 8 or 16 easily, but in principle can extend the scheme indefinitely.

Related Solutions

Electronic – How does TTL serial work

With TTL serial, there are two unidirectional data lines. Each is driven by the sender, both high and low. A 0 bit is represented by 0V a 1 bit by VCC.

The receiver's pin should be set to an input.

So, for a microcontroller to send a byte (8-N-1 no flow control) it could do something like this:

#define BAUDRATE 9600
#define DELAY (SYS_CLK/BAUDRATE)

#define UART_BITBANG_OFF     UART_BITBANG_PORT |= _BV(UART_BITBANG_PIN)
#define UART_BITBANG_ON      UART_BITBANG_PORT &= ~ _BV(UART_BITBANG_PIN)

#define UART_BITBANG_BIT(bit) {if (bit) UART_BITBANG_ON; else UART_BITBANG_OFF; _delay_us(DELAY);}

void uart_bitbang_init(void)
{
    UART_BITBANG_DDR &= ~ _BV(UART_BITBANG_PIN);        // TX output
}

void uart_bitbang_putc(uint8_t c)
{
    UART_BITBANG_BIT(1)
    UART_BITBANG_BIT((c & 0x1) == 0);
    UART_BITBANG_BIT((c & 0x2) == 0);
    UART_BITBANG_BIT((c & 0x4) == 0);
    UART_BITBANG_BIT((c & 0x8) == 0);
    UART_BITBANG_BIT((c & 0x10) == 0);
    UART_BITBANG_BIT((c & 0x20) == 0);
    UART_BITBANG_BIT((c & 0x40) == 0);
    UART_BITBANG_BIT((c & 0x80) == 0);
    UART_BITBANG_BIT(0);
}

(This code reads a bit backwards as it was originally meant for inverted TTL serial)

Of course, most MCUs have hardware UARTs which do all this for you.

Here's what you'd see on a scope:

https://www.pololu.com/docs/0J25/4.a

Here's a great video from ladyada explaining serial: http://www.adafruit.com/blog/2010/09/15/usb-serial-and-you-video-an-adafruit-after-school-special/

Electronic – Dual power supply confusing TTL serial

I would start with the 7 V to 5 V power regulator: there are zillions of regulators for that. You need an LDO (for Low Drop Out). If 500 mA is enough the LF50 is a good choice: drop out less than 0.5 V, ground current less than 0.5 mA. 2 % or 1 % accuracy.

As I understand you from the comments that infamous "cable" is a 50 cm wire, right? Then there should be no need for the two transistor inverters since the supply voltage for both microcontroller and TTL is 5 V. An AVR output can easily drive a TTL port half a meter further. You might replace the 74LS00 with a 74HC132 Schmitt-trigger input quad NAND if it's otherwise too noisy.

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Electronic – How does TTL serial work

Electronic – Dual power supply confusing TTL serial

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