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The specified maximum cable length with RS232 is 50 feet without an audio modem (modulator-demodulator) at each end. Most new computers today does not have a RS232 COM port which makes the use of a USB to serial converter useful for connecting equipment such as serial printers, scanners, scales and GPS devices, but also many types of business, office and laboratory equipment. However, how to apply each of the different serial interfacing options, including detailed wiring information for each approach, is presented near the end of this chapter. Then it references section 2.1.4 where the maximum capacitance of the receiver side of the interface point, including the cable, shall not exceed 2500pF. Then suggests seeing appendix A for guidance (again, the appendix states that it is not a formal part of the standard). This resistor will also make any length of cable, even a short length, look like the characteristic impedance of the cable (120 Ω in the model) to the driver.

In a RS-232 cable, data always flows in only one direction on any particular wire, from TX to RX. Other reasons that some RS-232 ports were half-duplex: some very old UARTs may be half-duplex limiting the system, and some very old computers drove the RS-232 drivers directly from the processor without a UART. This is not external biasing resistors, but a receiver characteristic of limiting induced differential noise due to imbalance of the inputs. This means that 5 V inputs are be fine so as long as the input current doesn't exceed the maximum current rating (IIIK). This connection may be used to limit the common-mode signal that can be impressed on the receiver inputs. A receipt response verifying valid data received can be sent back to the master. The PIC16F877 performs all the bookkeeping as to which particular bit the computer is sending and whether it is part of an address or data. Since all the TMC2209s share the same UART bus, this is used to give each one a unique address so they can be individually addressed. However, I wasn't going to get away that easily- Starfish needs two sensors and the XGZP6857D unfortunately has a fixed I2C address so they can't share the same I2C bus.

Since the PCA9545A provides four channels and I only needed two for the vacuum sensors, I had two leftover. Alternatively, with current-sourcing the SMINI provides the actual voltage that activates the railroad device. This provides easier system operation and debugging capability. The RS485 protocol employs a differential voltage system that allows it to operate effectively in environments with higher levels of electrical noise. These connect U1 to the SMINI's interface-unique circuitry for either RS232, or RS485 I/O. On-board RS485 input and output 120R resistors with built-in jumper caps for switching enable. The other two signals, RE and DE, enable the transceivers input and output respectively. The next part of the vacuum system is the two pneumatic solenoid valves. R1202 sets the peak current (Ipeak) used to actuate the solenoid. You can accomplish that current control scheme by driving a MOSFET using the PWM output of a microcontroller, but I wanted to try out the DRV120 - a single-channel relay, solenoid, and valve driver with current regulation.

The control board and feeders aren't very far apart so noise might not be a big enough issue on untwisted cables in practice. The solenoids I have are plenty hardy enough to handle 200 mA continuously so this is a perfectly reasonable setting. With the pumps I'm using it's barely necessary, but it's always good to have healthy safety margins. Using the SUSIC is the ideal approach for locations requiring large amounts of concentrated I/O such as at a lever-type CTC machine. For better or worse, RS485 standard a lot of external peripherals require 5 V power and I/O. R1401 is a pull-up resistor that keeps the motor turned off during power up. Since the DC motor is an inductive load this prevents the voltage spike that occurs when the motor is switched off from damaging the MOSFET. The minimum input and output voltage ratings may be exceeded if the input and output current ratings are observed. It's important that the ferrites don't get saturated so they're rated for nearly twice the maximum RMS current. 47 kΩ sets it to the maximum of 200 mA. These resistors determine the maximum motor current. The motor outputs need wide short traces connecting to the ESD circuitry and the connector.