Multiplexing TTL serial lines with transistors

Question

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

• Ignacio Vazquez-Abrams: What's wrong with using a `125?
• Spehro 'speff' Pefhany: What does The attached circuit works mean? Why not use 1/2 74HC4066 for each channel (assuming TTL or CMOS levels and CMOS inputs, and pull-down resistors on each input)?
• Pål Thingbø: I didn't know about 74HC4066. What do I gain using this, except for a little less soldering?

Answer 1

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.

• Pål Thingbø: Those are all NPN's in the schematic. Your design seems fine unless you have more than two slaves. Painting the same component (the slaves) twice in a schematic is bad design. Just sayin.
• WhatRoughBeast: Oops, you're right. I've edited.
• Ignacio Vazquez-Abrams: I wouldn't use LS with modern MCUs; too much to go wrong.
• Pål Thingbø: This is funny. On the MCU supplier's forum, people were trying to convince me to use software slave select (so you could have a single wire between all devices). I find that to be very complex when you have just 2 slaves, nevermind 10 in my plan. Also, if something goes wrong, you get a debugging hell, to say the least. Thanks for pointing me to the muxes, I'm implementing 74HC4066's.
• Pål Thingbø: About the current direction. The TX line is a driver, so my design is ok. You can even use CD4066 for this (<10mA) because MCU's have high impendance on the inputs.