Peter's electronic projects Peter's electronic projectsPBUS is an RS485-like multi-drop bus for interconnecting PIC and other microcontroller-driven devices. Protocol and software is written by Jap. Just include a separate pbus library file into your project and add your command handlers for the specific device in a few lines.
Software is in ALPHA stage, it may not even work for you. Every feedback is welcome, please send me E-mail or use the online comment system at the bottom of this page. I'd like to hear of you if you use this code for anything, have ideas, improvements of the source code.
There are three PIC source versions:
PBUS assigns a unique device ID in the range 1-f for every controller on the bus. The maximum number of slave devices on one bus is maximized in 15. Current protocol is one master, multiple slaves.
The implementation uses 2400 baud half-duplex serial communication. Only 2 wires (GND and PBUS signal) are needed to interconnect devices using their own power supply. Using a higher speed or other protocols on the same PBUS is possible, since slaves won't respond to packets they can't interpret.
Packets consist of 8-bit bytes. All packets contain at least 3 bytes. Maximum length is 18 bytes.
| byte offset | bits | name | description |
| 0 | 7-4 | DEVID | destination device PBUS ID |
| 0 | 3-0 | LEN | optional data length (0 means packet is 3 bytes, no optional data) |
| 1 | 7-0 | CMD | MASTER command or SLAVE response code |
| 2 | 7-0 | DATA[] | optional data, length is given in LEN (LEN=0 means no data) |
| 2+LEN | 7-0 | CHKSUM | packet checksum. Added to the packet bytes sum will result a sum of zero |
Master devices generates a 8500 usec gap on the bus before transmitting a PBUS packet. This is for synchronization of all listening receivers. Receivers can expect to see a min. 4900 usec, max. 21200 usec gap. Packet bytes must be sent without interruption. Packet reception will be reset if a gap longer than 200 usec is detected. After the request packet is sent, the master waits up to 1000 usec for a reply from the addressed slave. Slaves check received packets in this order:
PBUS handlers will implement some commands internally.
Commands implemented by ALL sources:
| dev | cmd code |
cmd parameters |
description |
| M | 5e | CVER | (check node version) |
| S | 60 | ROK <pbus version> [main version] [optional string describing node] | current nodes do not return the optional string by default |
| M | 5f | CPING [optional data] | (test if a node is present and the link is up) |
| S | 6f | RECHO [optional data echoed] | PB12 sources will not echo the supplied ping data back |
Commands implemented by sources except PB12:
| dev | cmd code |
cmd parameters |
description |
| M | 58 | CNOOP | (no operation) |
| S | 60 | ROK | |
| M | 5b | CLAST | (repeat last response) |
| S | (copy of the last response packet sent) | ||
| M | 5c | CRES | (reset statistics counters) |
| S | 60 | ROK | |
| M | 5d | CSTAT | (get PBUS statistics) |
| S | 60 | ROK <e1> <e2> <e3> [optional counters] | The CRES and CSTAT commands can be disabled. |
Meaning of counters:
| name | description |
| e1 | number of checksum errors |
| e2 | number of good packets received |
| e3 | number of good packets sent for me (not necessarily answered) |
The most simple case is when the controllers are close and you can directly wire them. If you use another pin than RA4 (open drain output) on the sw-UART 16x84 or use a hardware UART chip (eg 16F628), you need the diode on the TX pin as shown on the schematic, because devices can only drive the bus LOW, never HIGH. If you choose a hardware UART version, use the appropriate pins (RB1/RX, RB2/TX on 16F628).
For longer distances, it is recommended to use a smaller value pullup resistor on the bus and the usage of an external transistor.
A PC can participate on the PBUS as a slave or as a master device. Settings for the serial port are: 2400 bps, 8 bits, no parity, 1 stopbit, no flow control (XON/XOFF or RTS/CTS).
| name | version | description | download |
| PB12 | 023 | polling version for devices without interrupt (stack depth is 2) | DEMO written on 16F628, handles builtin commands. Not tested on 12c508 devices! |
| PBUS | 041 | software UART version for devices with interrupt | DEMO written on 16F628, handles builtin commands |
| PB628 | 068 | for devices with a free hardware UART | DEMO written on 16F628, handles builtin commands |
| pmaster | PBUS master controller polling devices on the bus | DEMO written on 16F628, polls 2-5 devices and displays statistics on LCD | |
| LCDLIB | 001 | library to drive an at least 2x16 LCD display see the lcdlib page for LCD interface schematic |
lcdlib.inc, lcdlib.asm are required for the demo sources to display debug info |