In part 2, we learned that:
- Only a small portion of RSS485 to TTL chips support automatic direction control. The majority of the chips need to be controlled by R̅E̅ and DE pin.
- The SP3485 uses much less power than the MAX13487 in a 3.3v setup.
- Most of the non-auto direction control modules you can buy, use a not gate as flow control.
Let’s take a closer look at how a not gate managed to control the flow direction in the XY-017 module. And then I modified the schematic a bit for easier understanding.
The TX signal goes through two not-gate and to DI pin. RO data also goes through two not-gate and to RX. The two not-gate could reform the signal and sharpen the edge, especially when we are driving an LED in this module.
The following oscilloscope graph recorded the signal of 74HC04 Pin 10 (which is the also same as pin DI) versus RS485+. The left part is the MCU asking for the holding resistor value and the right part is the reply from the sensor. There is a short voltage vibration during transitioning. The transition has to be located before the reply of the sensor. The sensor takes around 1ms before replying.
For the R̅E̅ and DI, the signal is latched by C4 (20nF ceramic cap). C4 is charged via R7 and discharged via D3 (there is no current flow into Pin 13), so the discharge rate is much faster than the charging rate. When Pin 10 is high, C4 gets charged. When Pin 10 is low, C4 discharges.
Take a closer look at pin 13 and the inverted output pin 12, we see that the 74ch04 inverts the signal when the input is around 1.3V.
The charging and discharge time has to be set up within the range for a correct latching time of R̅E̅ and DE. Or otherwise, the signal will be disturbed. For a too small capacitor, I changed the capacitor from 32nF to 1nF. R̅E̅ and DE changed state when the requesting message is not yet finished.
As a result, the 485+ 485- are pulled to neutral, and the difference between 485+ and 485- becomes zero (bus idling state). Although the sensor can still understand the message and make a reply.
And here is a zoomed-in capture.
If a too large capacitor is chosen, the transition will occur during the sensor reply signal and disturb the message. As a result, the MCU cannot understand the reply. ***In later time, I will switch to a Schmitt trigger inverter to check any improvement***
Auto direction control by transistor
On the other hand, we could use a PNP transistor as a not gate to control the flow.
MCU sending data
When TX is high, R̅E̅ and DE are low, SP3485 enters receiving mode. When TX is 1, the transistor activates, R̅E̅ and DE pull low, SP3485 output enters high-Z mode (high impedance mode), which means, the output is floating. When the output is floating, the bus line B- will be pulled down to ground while A+ pulled up to Vcc. So when TX is 1, A+ line is 1 and B- line is 0.
When TX is 0, the transistor is off, R̅E̅ and DE pull high. When DE is 1 and DI is 0, B- outputs 1, and A+ outputs 0. So when TX is 0, the 485 line is 0.
MCU receiving data
When the MCU is receiving data, the TX line is pulled 1, R̅E̅ and DE are 0, SP3485 enters receiving mode and transceives the 485 data to MCU.
| TXD | R̅E̅ | DE | DI(Pull GND) | B | A | Ro | |
| MCU sending | 1 | 0 | 0 | 0 | 0(Floating) | 1(Floating) | 1 |
| MCU sending | 0 | 1 | 1 | 0 | 1 | 0 | 0 |
| MCU receiving | 1 | 0 | 0 | 0 | 0(Floating) | 1(Floating) | 1 |
| MCU receiving | 1 | 0 | 0 | 0 | 1(Floating) | 0(Floating) | 0 |
Appendix: power consumption of Sipex SP3485 and Texas Instruments SN74HC04N
The following is using a Sipex SP3485 chip with Texas Instruments SN74HC04N hex inverter. The Sipex SP3485 does not feature a low power shutdown mode. So this module cannot drop to μA level consumption, please consider Sipex SP3481 or MaxLinear SP3485 if you need a low power mode. In this circuit board, all the pull-down and pull-up resistors are changed to 100k to minimize power loss.
Average transmission current: 4.47mA (requesting: 10.8mA, replying:653.7μA)
Average idling current: 182.86μA
However, the drawback of using such a weak pull-up is that the signal will be much noisier. The good news is that RS485 calculate the difference between AB line, as a result, they compensate each other.
