Need to "refurbish" the Vt1out of a MAX232 on a RS

Hi,

I am interfacing a vending-machine control board with a GSM module (a Sony-Ericsson GT47/48). The serial interfaces are RS232 (for the control-board) and TTL (for the module)

I have a MAX232 in the middle which works perfectly when tested between a PC and the module BUT it works intermittenly with the control-board, meannig that some times the control-board "hears" the module and sometimes not (on average 50-50), even though while sniffing the bus with an external PC the data looks clean.

The MAX232 has a voltage swing between 8.5v and -8.5v, which is quite inside the standard.

My Hypothesis is that the control-board expects 12v to -12v on the T1OUT. I tried using a 1458 opamp as a simple open loop non-inverting comparator to achieve that, but even though it improves the system it is far from a costumer grade product and it forces me to use a ±12v power supply.

I tested with several MAX232 ICs. with same results and I know the control-board is working OK.

Any idea on how to solve this problem?

Thanks in advance,

Seymour

Re: Need to "refurbish" the Vt1out of a MAX232 on

+/-8.5 is within RS-232 limts (+/-3 to +/-25), but if you suspect that you need at least +/-12V then you can try DS1488 (transmitter) and DS1489 (receiver). The transmitter operates of +/-15V.
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Regards,
IanP