Using a RS232/RS485 converter with a FP-1000 Network Interface.

[SuperSeptic]

I am attempting to use a Optically isolated RS232/RS485 converter to communicated to a FieldPoint FP-1000 Network Interface, and an RS-485 Serial Card. It is not working and I suspect the problem is with the RTS on the FP module. Is there a fix to this problem or am I forced to buy a FP-1001.

[Aaron_G]

Before you go and swap your FP-1000 for a FP-1001, there are a number of things to look at.

First of all, how is your RS-232 to RS-485 converter hooked up? By this I mean, its wiring to both the FP-1000 and the RS-485 port on the PC. Assuming that the Converted is the only thing wired to the RS-485 port on the PC, then you should be wiring Tx+ (PC) to Rx+ (converter), Tx- (PC) to Rx- (converter), Rx+ (PC) to Tx+ (converter) and Rx- (PC) to Tx- (converter). Between the converter and the FP-1000 (depending upon the converter's configuration), you may need either a straight cable or a null modem cable.

Secondly, you can use the FP-1000 to help you diagnose the problem. The FP-1000 has two LED's (well more than two but two we will focus only on two of them); Network
and Access. The Network LED should light whenever there is any communication on the serial port (whether the communication was addressed to the network module and it's attached I/O modules or anything else). The Access LED should only turn on when the network module or attached I/O modules are issued a command (read, write, change settings, etc...). Typically, if the Network LED remains lit at all times, it indicates mis-wiring of the serial lines.

Third, some converters require dip-switch settings for baud rate. Make sure the baud rate on the converter is the same being used by the software and that that value is the same as the FP-1000's dip-switches are set for (remember, for dip-switch setting changes to take affect, you need to cycle power to the FP-100x).

Also, the FP-1000 does not use hardware handshaking. It will assert the RTS and DSR signals for host computers, devices (such as modems) or software that may require these signals, but it does not look at the incoming handsh
ake lines.

Finally, if none of this solves the problem, please provide extra information on how exactly it is not working and we will see where we can go from there.

Regards,
Aaron

[SuperSeptic]

Aaron:
First thank you for quick and well thought out response. We were aware of all the items you mentioned. In addition, we checked out the RS-485 convertor module by wiring it from a RS-232 port on the computer to the RS-485 port on the plug in serial board, and checked it out with a COM test program provided by the manufacturer. All system were GO! We also checked out the RS-232 conection by going directlyfrom the FP-1000 module to the RS-232 port on the computer, all the lights blinked and communication was established. But when we connected up using the RS-485 module, no communication. My question about the RTS line was whether it asserts before the data is sent and if it is set low after the last data bit is sent. The timing is important and the s
erial card includes special software to automatically adjust its timing to the baud rate and remains enabled for the transmission time and ten data bits after data transfer is completed. Here in lies the problem. Any suggestions?