Termination resistors also reduce electrical noise sensitivity due to the lower impedance. This allows for basic error detection, in that if noise on the transmission line causes one bit to be received incorrectly, either received as a '0' when transmitted as a '1' or vice-versa, the error would be detected due to the count of '1' bits in the byte being odd when it is expected to be even, or vice-versa depending on the parity checking settings. For seven data bits with a parity bit, M would be cleared (equal to zero), and PE would be set in order to make the most-significant bit of a normal eight-bit byte be used by the serial port as a parity bit. Because we chose the default baud rate (which the terminal is presumably already set for), you can simply move the serial cable from the Serial Port 1 connector to the Serial Port 2 connector on the Docking Panel to complete the change to the new port.

By default, the RS485 connections are not brought out to the Docking Panel’s DB-9 Serial1 Connector. In this case, cable connections may be made to Serial 2 on either the 10-pin PDQ Board Serial Communications Header, or the Docking Panel’s 10-pin right-angle Serial Header, or the Docking Panel’s Serial2 DB-9 Connector. Because all of the serial I/O routines on the PDQ Board are revectorable, it is very easy to change the serial port in use without modifying any high level code. The terminal program communicates with the PDQ Board via this serial port. In this case, cable connections may be made to Serial 2 at pins 4 and 10 of the PDQ Board’s 10-pin Serial Header, or pins 5 and 6 of the Docking Panel’s 10-pin right-angle Serial Header. You can use one or both of the PDQ Board’s RS485 links to create such a multi-drop serial network. We assume that you are now communicating with the PDQ Board via the default Serial1 port at the standard 115200 baud rate.

Serial1 port the default startup serial link. In the most common multi-drop RS485 protocol, one computer is designated as a master and the rest of the computers or devices on the serial bus are designated as slaves. Although the devices would share the same network, communications would only be understandable by members of the same group. To ensure that no two devices drive the network at the same time, it is necessary that each slave device be able to disable its own RS485 data transmitter. If more than one slave tried to drive the transmit line simultaneously, their serial drivers would fight with each other for control of the bus. Multiple receivers may be connected to such a network in a linear, multidrop bus. RS485 multi-drop networks are daisy-chained networks with a single cable connecting multiple devices. Bias termination - Using resistive termination decreases noise immunity, particularly if the cable is loaded with many devices. No termination - If the PDQ Board is not an end device, you should not terminate that cable. Resistive termination - If the PDQ Board is at the end of the RS485 cable you can terminate the cable by installing jumper caps at both jumper locations, "Term" and "RTerm".

In that case, when using very long cables you can improve noise immunity and assure a valid idle level when the transceiver is not active by installing bias resistors. RS485Receive() clears bit PD5 to place the transceiver in receive mode, and RS485Transmit() sets bit PD5 to place the transceiver in transmit mode. 1 or 2 to specify Serial1 or Serial2, respectively, and clears the appropriate PORTJ bit to place the transceiver in receive mode. Done that establishes the RS485 receive mode is coded such that it waits until all queued (pending) characters have been transmitted before the driver chip is taken out of transmit mode. If PT is cleared, then all transmitted bytes with a parity bit will have an even number of total '1' bits. This is an extra single bit appended to the end of each byte or character transmitted, which is set or cleared as necessary to ensure that the total number of '1' bits in the byte is always odd or even. When PE is cleared (equal to zero), the most-significant bit of each transmitted character will be a data bit.