Serial Ports: Difference between revisions
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Serial ports are a legacy communications port common on IBM-PC compatible computers. Use of serial ports for connecting peripherals has largely been deprecated in favor of [[USB]] and other modern peripheral interfaces, however it is still commonly used in certain industries for interfacing with industrial hardware such as CNC machines or commercial devices such as POS terminals. Historically it was common for many dial-up modems to be connected via a computer's serial port, and the design of the underlying [[UART]] hardware itself reflects this.
Serial ports are typically controlled by [[UART]] hardware. This is the hardware chip responsible for encoding and decoding the data sent over the serial interface. Modern serial ports typically implement the [[wikipedia:RS-232|RS-232]] standard, and can use a variety of different connector interfaces. The DE-9 interface is the one most commonly used connector for serial ports in modern systems.
Serial ports are of particular interest to operating-system developers since they are much easier to implement drivers for than USB, and are still commonly found in many x86 systems. It is common for operating-system developers to use a system's serial ports for debugging purposes, since they do not require sophisticated hardware setups and are useful for transmitting information in the early stages of an operating-system's initialization. Many emulators such as [[QEMU]] and [[Bochs]] allow the redirection of serial output to either stdio or a file on the host computer.
== Wires, Pins, Connectors and the like ==
The Wikipedia page on [[wikipedia:Serial_port|Serial ports]] has a lot of information, and it is summarised here. The serial interface is very simple. There are actually two kinds of serial port: 25-pin and 9-pin. 25-pin ports are not any better, they just have more pins (most unused) and are bigger. 9-pin is smaller and is used more often though in the past the 25-pin ones were used more often. The 9-pin ones are called DE-9 (or more commonly, DB-9 even though DE-9 is
Both have the same basic types of pins. A DB-25 has most of the pins as ground pins or simply unconnected, whereas a DE-9 has only
If you want to connect two computers, you need two things in your cable:
# The cable needs to have two female plugs so it can plug into both computers.
# The cable needs to have
For serial devices, you don't need to setup the cable this way. The receiving end of the device has the wires switched and it has a female port, which means you can plug a male plug into it.
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During the early stages of kernel development, you might wonder why you would bother writing a serial driver. There are several reasons why you might:
;[[GDB]] debugging
:You can use the serial port to connect to a host computer, and use the GDB debugger to debug your operating system. This involves writing a stub for GDB within your OS
;Headless console
:You can operate the computer without a monitor, keyboard or mouse and instead use the serial port as a console using a protocol such as TTY or VT100.
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If you want to use the serial port for communications, you first have to initialize it. You tell it how fast your connection speed between the other computer or device will be (this is called the baud rate)--you must have the same speed as the other device or computer is setup to use, or you will have problems. It is probably safer to use the slower speeds unless you need the faster speeds for some reason, for example if you are playing a multi-player game over a serial connection. You also need to setup the parity type and the number of bits in a character. Once again, your computer must be setup with the same values for these things as the other computer or device has, or communication will not work.
Once you have setup these things, you still need to setup the interrupt handlers. You can poll the port to see if any new things have arrived, or if it's time to send another character, but this slows things down and will not work very well in most real-time applications or
You use IRQ #4 for COM ports 1 or 3, and IRQ #3 for COM ports 2 or 4 (you can tell which port sent the interrupt when you receive the interrupt). The IRQ handlers check if you are receiving something, and if so they receive the character and handle it somehow, such as placing it into a buffer. They also check if the other side is ready to receive something from you, and if you have something to send, it is sent.
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The addresses for COM ports can vary depending on how they are connected to the machine and how the BIOS is configured. Some BIOS configuration utilities allow you to see and set what these are, so if you in doubt for a test machine, this might be a good place to look to get you started.
For the most part, the first two COM ports will be at the addresses specified, the addresses for further COM ports
{| {{wikitable}}
! COM Port
! IO Port
|-
| COM1 ||
|-
| COM2 ||
|-
| COM3 ||
|-
| COM4 ||
|-
| COM5 || 0x5F8
|-
| COM6 || 0x4F8
|-
| COM7 || 0x5E8
|-
| COM8 || 0x4E8
|}
You might be able to find the IO port addresses of the COM ports in the [[Memory Map (x86)#BIOS_Data_Area_.28BDA.29|BIOS Data Area]]; however be warned that this won't work on modern/UEFI systems, can tell you about serial ports that only exist in the chipset (and lack any kind of connector that anything can be plugged into), won't tell you about any additional serial ports (e.g. on expansion cards, etc) that firmware doesn't/can't know about, and will make your OS susceptible to "BIOS quirks/bugs". Because the serial ports have relatively standard IO ports it's far more effective to use manual probing techniques instead; specifically, see if the scratch pad register can store a value, then try the loopback test (that you should use to determine if the serial port is faulty anyway).
Once you have the base address of your COM port, you add an offset value to get to one of the data registers. One of the registers hold what is termed the DLAB or Divisor Latch Access Bit. When this bit is set, offsets 0 and 1 are mapped to the low and high bytes of the Divisor register for setting the baud rate of the port. When this bit is clear, offsets 0 and 1 are mapped to their normal registers. The DLAB bit only affects port offsets 0 and 1, the other offsets ignore this setting.
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! IO Port Offset
! Setting of DLAB
! I/O Access
! Register mapped to this port
|-
| +0 || 0 ||
|-
| +
|-
| +1 || 0 || Read/Write || Interrupt Enable Register.
|-
| +
|-
| +1 || 1 || Read/Write || With DLAB set to 1, this is the most significant byte of the divisor value.
|-
| +2 || - || Read || Interrupt Identification
|-
| +
|-
| +
|-
| +
|-
| +
|-
| +6 || - || Read || Modem Status Register.
|-
| +7 || - || Read/Write || Scratch Register.
|}
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# Send the most significant byte of the divisor value to [PORT + 1].
# Clear the most significant bit of the Line Control Register.
====Line Control Register====
The Line Control register sets the general connection parameters.
{| class="wikitable"
|-
! Bit 7
! Bit 6
! Bits 5-3
! Bit 2
! Bits 1-0
|-
| Divisor Latch Access Bit
| Break Enable Bit
| Parity Bits
| Stop Bits
| Data Bits
|}
====Data Bits====
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|}
====Stop
The serial controller can be configured to send a number of bits after each character of data. These reliable bits can be used to by the controller to verify that the sending and receiving devices are in phase.
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|}
====Parity Bits====
The controller can be made to add or expect a parity bit at the end of each character of data transmitted. With this parity bit, if a single bit of data is inverted by interference, a parity error can be raised. The parity type can be NONE, EVEN, ODD, MARK or SPACE.
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To set the port parity, set bits 3, 4 and 5 of the Line Control Register [PORT + 3].
{| {{wikitable}}
! Bit 5
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To communicate with a serial port in interrupt mode, the interrupt-enable-register (see table above) must be set correctly. To determine which interrupts should be enabled, a value with the following bits (0 = disabled, 1 = enabled) must be written to the interrupt-enable-register:
{| {{wikitable}}
! Bit 7-4
! Bit 3
! Bit 2
! Bit 1
! Bit 0
|-
| Reserved
| Modem Status
| Receiver Line Status
| Transmitter Holding Register Empty
| Received Data Available
|}
===First In First Out Control Register===
The First In / First Out Control Register (FCR) is for controlling the FIFO buffers. Access this register by writing to port offset +2.
{| class="wikitable"
|-
! Bits 7-6
! Bits 5-4
! Bit 3
! Bit 2
! Bit 1
! Bit 0
|-
| Interrupt Trigger Level
| Reserved
| DMA Mode Select
| Clear Transmit FIFO
| Clear Receive FIFO
| Enable FIFO's
|}
====Clear Transmit FIFO and Clear Receive FIFO====
Bit 2 being set clears the Transmit FIFO buffer while Bit 1 being set clears the Receive FIFO buffer. Both bits will set themselves back to 0 after they are done being cleared.
====Interrupt Trigger Level====
The Interrupt Trigger Level is used to configure how much data must be received in the FIFO Receive buffer before triggering a Received Data Available Interrupt.
{| {{wikitable}}
! Bit 7
! Bit 6
! Trigger Level
|-
| 0 || 0 || 1 Byte
|-
| 0 || 1 || 4 Bytes
|-
| 1 || 0 || 8 Bytes
|-
| 1 || 1 || 14 Bytes
|}
===Interrupt Identification Register===
The Interrupt Identification Register (IIR) is for identifying pending interrupts. Access this register by reading from port offset +2.
{| class="wikitable"
|-
! Bits 7-6
! Bits 5-4
! Bit 3
! Bit 2-1
! Bit 0
|-
| FIFO Buffer State
| Reserved
| Timeout Interrupt Pending (UART 16550) or Reserved
| Interrupt State
| Interrupt Pending
|}
====Interrupt State====
After Interrupt Pending is set, the Interrupt State shows the interrupt that has occurred. They have varying levels of priority, with high-value interrupts handled first, and low-value interrupts being handled last.
{| {{wikitable}}
! Bit 2
! Bit 1
! Interrupt
! Priority
|-
| 0 || 0 || Modem Status || 4 (Lowest)
|-
| 0 || 1 || Transmitter
|-
| 1 || 0 || Received Data Available || 2
|-
|
|}
====FIFO Buffer State====
{| {{wikitable}}
! Bit 7
! Bit 6
! State
|-
| 0 || 0 || No FIFO
|-
| 0 || 1 || FIFO Enabled but Unusable
|-
|
|}
===Modem Control Register===
The Modem Control Register is one half of the hardware handshaking registers.
While most serial devices no longer use hardware handshaking, The lines are still included in all 16550 compatible UARTS.
These can be used as general purpose output ports, or to actually perform handshaking.
By writing to the Modem Control Register, it will set those lines active.
{| {{wikitable}}
! Bit
! Name
! Meaning
|-
| 0 || Data Terminal Ready (DTR) || Controls the Data Terminal Ready Pin
|-
| 1 || Request to Send (RTS) || Controls the Request to Send Pin
|-
| 2 || Out 1 || Controls a hardware pin (OUT1) which is unused in PC implementations
|-
| 3 || Out 2 || Controls a hardware pin (OUT2) which is used to enable the IRQ in PC implementations
|-
| 4 || Loop || Provides a local loopback feature for diagnostic testing of the UART
|-
| 5 || 0 || Unused
|-
| 6 || 0 || Unused
|-
| 7 || 0 || Unused
|}
Most PC serial ports use OUT2 to control a circuit that disconnects (tristates) the IRQ line. This makes it possible for multiple serial ports to share a single IRQ line, as long as only one port is enabled at a time.
Loopback mode is a diagnostic feature. When bit 4 is set to logic 1,
the following occur the transmitter Serial Output (SOUT) is set to the Marking (logic 1) state; the receiver Serial
Input (SIN) is disconnected; the output of the Transmitter Shift Register is ‘‘looped back’’ into the Receiver Shift
Register input; the four MODEM Control inputs (DSR, CTS, RI, and DCD) are disconnected; and the four
MODEM Control outputs (DTR, RTS, OUT 1, and OUT 2) are internally connected to the four MODEM Control
inputs, and the MODEM Control output pins are forced to their inactive state (high). In the loopback mode, data
that is transmitted is immediately received. This feature allows the processor to verify the transmit-and received-
data paths of the UART. In the loopback mode, the receiver and transmitter interrupts are fully operational. Their sources are external to
the part. The MODEM Control Interrupts are also operational, but the interrupts’ sources are now the lower four
bits of the MODEM Control Register instead of the four MODEM Control inputs. The interrupts are still controlled
by the Interrupt Enable Register.
===Line Status Register===
The line status register is useful to check for errors and enable polling.
{| {{wikitable}}
! Bit
! Name
! Meaning
|-
| 0 || Data ready (DR) || Set if there is data that can be read
|-
| 1 || Overrun error (OE) || Set if there has been data lost
|-
| 2 || Parity error (PE) || Set if there was an error in the transmission as detected by parity
|-
| 3 || Framing error (FE) || Set if a stop bit was missing
|-
| 4 || Break indicator (BI) || Set if there is a break in data input
|-
| 5 || Transmitter holding register empty (THRE) || Set if the transmission buffer is empty (i.e. data can be sent)
|-
| 6 || Transmitter empty (TEMT) || Set if the transmitter is not doing anything
|-
| 7 || Impending Error || Set if there is an error with a word in the input buffer
|}
===Modem Status Register===
This register provides the current state of the control lines from a peripheral device.
In addition to this current-state information, four bits of the MODEM Status Register provide change information.
These bits are set to a logic 1 whenever a control input from the MODEM changes state. They are reset to logic
0 whenever the CPU reads the MODEM Status Register
{| {{wikitable}}
! Bit
! Name
! Meaning
|-
| 0 || Delta Clear to Send (DCTS) || Indicates that CTS input has changed state since the last time it was read
|-
| 1 || Delta Data Set Ready (DDSR) || Indicates that DSR input has changed state since the last time it was read
|-
| 2 || Trailing Edge of Ring Indicator (TERI) || Indicates that RI input to the chip has changed from a low to a high state
|-
| 3 || Delta Data Carrier Detect (DDCD) || Indicates that DCD input has changed state since the last time it ware read
|-
| 4 || Clear to Send (CTS) || Inverted CTS Signal
|-
| 5 || Data Set Ready (DSR) || Inverted DSR Signal
|-
| 6 || Ring Indicator (RI) || Inverted RI Signal
|-
| 7 || Data Carrier Detect (DCD) || Inverted DCD Signal
|}
If Bit 4 of the MCR (LOOP bit) is set, the upper 4 bits will mirror the 4 status output lines set in the Modem Control Register.
===Terminals===
{{Main|Terminals}}
Once you can send and receive bytes with confidence, you probably want to connect the serial port to a terminal (or more likely a terminal emulator these days). Those send specific byte sequences when a key is pressed, and can interpret codes to move the cursor on the screen and change color for example.
==Example Code==
===Initialization===
<
#define PORT 0x3f8
outb(PORT + 1, 0x00); // Disable all interrupts
outb(PORT + 3, 0x80); // Enable DLAB (set baud rate divisor)
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outb(PORT + 2, 0xC7); // Enable FIFO, clear them, with 14-byte threshold
outb(PORT + 4, 0x0B); // IRQs enabled, RTS/DSR set
outb(PORT + 4, 0x1E); // Set in loopback mode, test the serial chip
outb(PORT + 0, 0xAE); // Test serial chip (send byte 0xAE and check if serial returns same byte)
// Check if serial is faulty (i.e: not same byte as sent)
if(inb(PORT + 0) != 0xAE) {
return 1;
}
// If serial is not faulty set it in normal operation mode
// (not-loopback with IRQs enabled and OUT#1 and OUT#2 bits enabled)
outb(PORT + 4, 0x0F);
return 0;
}
</syntaxhighlight>
Notice that the initialization code above writes to [PORT + 1] twice with different values. This is once to write to the Divisor register along with [PORT + 0] and once to write to the Interrupt register as detailed in the previous section. The second write to the Line Control register [PORT + 3] clears the DLAB again as well as setting various other bits.
===Receiving data===
<
int serial_received() {
return inb(PORT + 5) & 1;
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return inb(PORT);
}
</syntaxhighlight>
===Sending data===
<
int is_transmit_empty() {
return inb(PORT + 5) & 0x20;
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outb(PORT,a);
}
</syntaxhighlight>
==Glossary==
;Baud Rate
:
:The fastest baud rate a serial port can reliably run at is generally 115200 baud.
;Baud Rate Divisor
:The value the is used by the UART to divide its internal clock by in order to get the actual intended baud rate.
;Stop Bits
:The NULL bit(s) sent between each character to synchronize the transmitter and the receiver.
;UART
:
== Related Links ==
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[[Category:Network Hardware]]
[[Category:Common Devices]]
[[de:Serielle_Schnittstelle]]
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