Post by thread:[PICLIST] [PIC] RS232 BUS ?

On Mon, Jul 02, 2001 at 09:37:19PM +0200, Patrick J wrote: > Hi, I am investigating the possibillity to have 4 PIC 16F877 connected > to the same PC on a RS232 'bus'. > I am using MAX220CPE, can they be connected in parallell or will they go > up in flames ? Yes. RS-232 is point to point. If you want a bus configuration you should take a look at RS-485. If you really want RS-232, instead of using a bus configuration, instead wire in a ring. So instead of: +------------+-------------+------------+----------------+ <- RS232 | | | | | ------- ------- ------- ------- ------- | PC | |Node1| |Node2| |Node3| |Node4| ------- ------- ------- ------- ------- Do this: ------- ------- ------- ------- ------- | PC |----->|Node1|------>|Node2|----->|Node3|-------->|Node4|--v ------- ------- ------- ------- ------- | ^ | +--------------------------------------------------------------+ Where the PC talks to Node 1, Node 1 to Node 2, and so forth. It will be slower than the bus, but works with the technology. BAJ -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics

If all of the following are true: - The PC is going to act as a master - The F877's will only speak when spoken to - The F877's are physically close to each other (ideally sharing the same power supply). Then there is a low-cost simple way to be your network... Take the TD signal from the PC and route it thru the RS232 to TTL channel of the MAX220 to the RD signals of all four PICS. Connect a signal diode (IN4148 is fine) from the TD pin of each of the PICs to the TTL to RS232 channel of the MAX220. The cathode of the diode should be connected to the PIC pin. Connect a 10k resistor from the junction of all the diodes with the MAX220 pin to the positive supply rail. Connect the output of the TTL to RS232 channel of the MAX220 to the PC. All the F877's will be able to 'hear' the PC. The PC will be able to 'hear' any F877. As long as only one F877 at a time 'talks' everything will be well. If more than one F877 tries to talk at the same time the PC will get garbled data, but no hardware damage will be done. Bob Ammerman RAm Systems (contract development of high performance, high function, low-level software) -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics

You can also do a cheap short distance serial bus using TTL levels right out of the PIC. In a dental product, I have a couple PICs in the desktop unit, then another in the handheld unit. The handheld device does two way communications with the desktop using three wires (+5V, ground, data). At each end, the data line goes through a current limit resistor to the receive input of the PIC. A diode is between the transmit pin of the PIC and the receive pin such that the transmit pin can pull the receive pin down, but not up. A pull-up resistor is in the desktop unit from the data line to +5V. The current limit resistor is small enough so we do not get significant drop across it during normal operation. Communications is packet based, with each end having an address. This way each end knows to ignore data that it has sent and only receive data from the other end. This is a polled system where the desktop sends a packet and the handheld unit responds with a packet. The handheld unit is receiving two data bytes (representing two digits on a 7 segment display - one bit is one segment). The handheld unit transmits one byte representing the state of two push button switches. I'm also looking at another application where we'd use ALOHA sharing of the bus. Here, one unit receives data from several transmitters. Each PIC drives an opto isolator that has an open collector output. These are all tied together with a pull-up resistor at the receiver. Each transmitter randomly transmits its data. Due to the short data burst and the long period of time between transmissions, we assume the bus is available (similifying hardware). If we miss a packet now and then due to contention, no harm is done. The lack of a receiver on the transmit units simplifies the hardware since we need the opto isolation for the transmitting units. Harold On Mon, 2 Jul 2001 17:21:09 -0400 Byron A Jeff <spam_OUTbyronTakeThisOuTCC.GATECH.EDU> writes: {Quote hidden}> On Mon, Jul 02, 2001 at 09:37:19PM +0200, Patrick J wrote: > > Hi, I am investigating the possibillity to have 4 PIC 16F877 > connected > > to the same PC on a RS232 'bus'. > > I am using MAX220CPE, can they be connected in parallell or will > they go > > up in flames ? > > Yes. RS-232 is point to point. If you want a bus configuration you > should > take a look at RS-485. > > If you really want RS-232, instead of using a bus configuration, > instead > wire in a ring. So instead of: > > +------------+-------------+------------+----------------+ <- > RS232 > | | | | | > ------- ------- ------- ------- ------- > | PC | |Node1| |Node2| |Node3| |Node4| > ------- ------- ------- ------- ------- > > Do this: > > ------- ------- ------- ------- ------- > | PC |----->|Node1|------>|Node2|----->|Node3|-------->|Node4|--v > ------- ------- ------- ------- ------- | > ^ | > +--------------------------------------------------------------+ > > Where the PC talks to Node 1, Node 1 to Node 2, and so forth. > > It will be slower than the bus, but works with the technology. > > BAJ > > -- > http://www.piclist.com hint: The list server can filter out > subtopics > (like ads or off topics) for you. See http://www.piclist.com/#topics > > FCC Rules Online at http://hallikainen.com/FccRules Lighting control for theatre and television at http://www.dovesystems.com ________________________________________________________________ GET INTERNET ACCESS FROM JUNO! Juno offers FREE or PREMIUM Internet access for less! Join Juno today! For your FREE software, visit: dl.http://www.juno.com/get/tagj. -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.

Yes, this was also suggested by Bob Ammerman and it sounds just like the way to go for my purpose: 4 sensors connected to 4 PICs so parameters can be adjusted aut. by the PC. Actual output from the sensors wont go through RS232 since they must be read in sync and responstime is an issue. I have also looked into RS485 as Byron refered to in his reply. I am not sure how that works, but I found some RS485 tranciver ICs. Speed is 250kbps to 4mbps. Does 485 use some special protocol, or can I just the TX/RX from the PIC to the 485 tranciver ? PIC1 <-> RS485 tranciver <==== 485 BUS ====> <- RS485 tranciver <-> PIC2 /PJ From: "Harold M Hallikainen > You can also do a cheap short distance serial bus using TTL levels right > out of the PIC. In a dental product, I have a couple PICs in the desktop [snip] > > Hi, I am investigating the possibillity to have 4 PIC 16F877 > > connected to the same PC on a RS232 'bus'. [snip] -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email .....listservKILLspam@spam@mitvma.mit.edu with SET PICList DIGEST in the body

On Wed, Jul 04, 2001 at 07:46:37AM +0200, Patrick J wrote: > Yes, this was also suggested by Bob Ammerman and it sounds just like the way > to go for my purpose: 4 sensors connected to 4 PICs so parameters can be > adjusted aut. by the PC. Actual output from the sensors wont go through > RS232 since they must be read in sync and responstime is an issue. Well as Bob pointed out, as long as the parts are in close proximity, that wire-or TTL can do the job. > > I have also looked into RS485 as Byron refered to in his reply. I am not sure > how that works, but I found some RS485 tranciver ICs. Speed is 250kbps > to 4mbps. Don't worry about the speed. The real bottom line of RS-485 is the fact that it's true multistation. Up to 32 nodes can be connected to the same pair of lines. > > Does 485 use some special protocol, or can I just the TX/RX from the PIC > to the 485 tranciver ? > PIC1 <-> RS485 tranciver <==== 485 BUS ====> <- RS485 tranciver <-> PIC2 Exactly as you describe. The only catch is that only one transmitter can be selected at a time. This has a couple of hardware and software implications: Hardware: Each node, including the PC, must have a way to select the direction of the transceiver. For the PICs it's fairly simple: use another I/O pin for direction control. However for PC's it's a bit tougher. The easiest way to do it is to have a one-shot that selects transmit whenever the TX goes low and times out after two character times or so. So transmit is selected as long as you're transmitting characters, then it automatically reverts to receive once transmit ends. Software: Add code to select the direction. Start in receive, wait until the PC talks to you then transmit. That's the basic protocol. BAJ {Quote hidden}> > /PJ > > From: "Harold M Hallikainen > > You can also do a cheap short distance serial bus using TTL levels > right > > out of the PIC. In a dental product, I have a couple PICs in the desktop > [snip] > > > > Hi, I am investigating the possibillity to have 4 PIC 16F877 > > > connected to the same PC on a RS232 'bus'. > [snip] > > -- > http://www.piclist.com#nomail Going offline? Don't AutoReply us! > email listservKILLspammitvma.mit.edu with SET PICList DIGEST in the body > -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email .....listservKILLspam.....mitvma.mit.edu with SET PICList DIGEST in the body

On Wed, 4 Jul 2001 07:46:37 +0200 Patrick J <EraseMEinfospam_OUTTakeThisOuTDATECH.SE> writes: {Quote hidden}> Yes, this was also suggested by Bob Ammerman and it sounds just like > the way > to go for my purpose: 4 sensors connected to 4 PICs so parameters > can be > adjusted aut. by the PC. Actual output from the sensors wont go > through > RS232 since they must be read in sync and responstime is an issue. > > I have also looked into RS485 as Byron refered to in his reply. I am > not sure > how that works, but I found some RS485 tranciver ICs. Speed is > 250kbps > to 4mbps. > > Does 485 use some special protocol, or can I just the TX/RX from the > PIC > to the 485 tranciver ? > PIC1 <-> RS485 tranciver <==== 485 BUS ====> <- RS485 tranciver <-> > PIC2 > The EIA485 transceivers just handle the electrical part of the job. It's your job to figure out a protocol. You have to drive the enable pin on each transmitter at the appropriate time so you don't get contention on the bus. The simplest method is using polling. Have a master unit poll each slave unit. The slaves transmit only when asked. I also use slew rate limited chips (like the MAX488). These help reduce transmission line reflections. You also need to use cable with the appropriate characteristic impedance (typically 110 ohms) and terminate each end of the cable. The Maxim datasheets have lots of good info on this. For short low speed runs, though, I'd just do an "open collector bus." Harold ________________________________________________________________ GET INTERNET ACCESS FROM JUNO! Juno offers FREE or PREMIUM Internet access for less! Join Juno today! For your FREE software, visit: dl.http://www.juno.com/get/tagj. -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads

> Hi, I am investigating the possibillity to have 4 PIC 16F877 connected > to the same PC on a RS232 'bus'. > I am using MAX220CPE, can they be connected in parallell or will they go > up in flames ? The only 'official' way to do this (assuming you want the PC to talk to the PICs and the PICs to the PC, not PICs to PICs) is to daisy chain the devices: Downstreams from the PC each devices converts the V24 to TTL (to feed the PIC) and the TTL back to V24 (to feed the next device). Upstreams each device converst what it receives from the previous device to TTL, adds (or substracts, check the polarity) its own signal, and converts it to V24 again for the next device or the PC. When I have the time I'll modify WISP/Wbus to use this principle, seem the current Wisp/Wbus for a cheaper but much less reliable way to chain V24 devices. Wouter -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads